:root{color-scheme:dark;--bg:#071321;--panel:#12283b;--panel2:#19364d;--border:#36536a;--mint:#55dfbc;--text:#eaf3fa;--muted:#a3bac9;--danger:#ffd0b7;--safe-top:env(safe-area-inset-top,0px);--safe-bottom:env(safe-area-inset-bottom,0px)}*{box-sizing:border-box}html,body{margin:0;min-height:100%;background:var(--bg);color:var(--text);font-family:system-ui,-apple-system,"Segoe UI",sans-serif;-webkit-tap-highlight-color:transparent}button{font:inherit;color:inherit;cursor:pointer;touch-action:manipulation}button:focus-visible{outline:3px solid var(--mint);outline-offset:2px}button:disabled{opacity:.48;cursor:default}[hidden]{display:none!important}.app{max-width:760px;margin:auto;padding-bottom:calc(28px + var(--safe-bottom))}.app-head{height:64px;display:flex;align-items:center;justify-content:space-between;border-bottom:1px solid #283f52;padding:8px 14px}.brand{display:flex;align-items:center;gap:10px}.brand-icon{width:37px;height:37px;display:grid;place-items:center;font-size:28px;border-radius:13px;background:#15423b;color:var(--mint)}.brand b{font-size:16px;letter-spacing:.06em}.brand small{display:block;color:var(--muted);font-size:10px;margin-top:1px;letter-spacing:.075em}.head-btn{border:1px solid var(--border);border-radius:10px;background:var(--panel);width:37px;height:37px}.main{padding:12px 12px 40px}.taskbar{display:flex;gap:8px;align-items:center;justify-content:space-between;margin-bottom:10px}.eyebrow{font-weight:750;font-size:10px;color:var(--mint);letter-spacing:.07em}.task-picker{display:flex;gap:8px;align-items:center;justify-content:center;padding:9px 10px;min-height:39px;border:1px solid var(--border);border-radius:10px;background:var(--panel);font-size:13px;font-weight:700;max-width:68%}.task-picker span:nth-child(1){overflow:hidden;white-space:nowrap;text-overflow:ellipsis}.workspace{border-radius:19px;background:linear-gradient(155deg,#1b3c53,#10283e);border:1px solid #3c6e7e;padding:13px 12px 12px;min-height:185px;box-shadow:0 7px 23px #0002}.workspace-top{display:flex;align-items:center;justify-content:space-between;gap:5px}.work-label{font-size:11px;font-weight:800;letter-spacing:.07em;color:var(--mint)}.work-count{font-size:11px;color:var(--muted)}.equation{display:flex;align-items:center;justify-content:center;gap:5px;min-height:88px;max-width:100%;flex-wrap:wrap;overflow-wrap:anywhere}.eq-side{display:flex;gap:3px;align-items:center;justify-content:center;flex-wrap:wrap;min-width:0}.eq-sign{font-size:21px;color:var(--mint);font-weight:700}.eq-chip,.coef-chip{background:#214d51;border:1px solid #60bba8;border-radius:11px;min-height:49px;min-width:37px;padding:7px 9px;display:inline-flex;align-items:center;justify-content:center;white-space:normal;font-family:"Cambria Math",Georgia,serif;font-size:clamp(17px,5vw,26px);line-height:1.17;overflow-wrap:anywhere}.eq-chip.selected,.coef-chip.selected{outline:2px solid #f8d57a;outline-offset:2px;background:#3f4f47}.coef-chip{font-size:clamp(15px,4.3vw,22px)}.multiply-text{font-size:20px}.var-text{font-size:21px}.eq-zero{color:var(--text);font-family:Georgia,serif;font-size:22px}.eq-chip math{font-size:clamp(17px,5vw,24px)}.selection-line{font-size:12px;line-height:1.38;color:#d2e4ec;border-top:1px solid #ffffff22;padding-top:8px;min-height:27px}.notice{font-size:12px;line-height:1.4;min-height:0;color:var(--mint);overflow-wrap:anywhere}.notice.error{color:var(--danger)}.work-actions{margin-top:10px;padding:10px;border:1px solid #2e5265;background:#102338;border-radius:17px}.action-label{display:flex;align-items:center;justify-content:space-between;gap:5px;color:#d2e8ee;font-size:12px;margin:0 0 7px;min-height:26px}.action-label>span{min-width:0;overflow:hidden;text-overflow:ellipsis;white-space:nowrap}.action-label button{background:#24475d;border:1px solid #467a84;border-radius:8px;min-width:78px;min-height:32px;font-weight:700;font-size:12px}.ops{display:grid;grid-template-columns:repeat(4,minmax(0,1fr));gap:7px}.ops>button{min-width:0;min-height:72px;display:flex;flex-direction:column;align-items:center;justify-content:center;gap:1px;background:#204056;border:1px solid #548095;border-radius:13px}.ops strong{font-size:30px;font-weight:700;color:var(--mint);line-height:1}.ops small{font-size:clamp(9px,2.65vw,12px)}.sub-actions{display:grid;grid-template-columns:1.2fr 1fr 1fr;gap:7px;margin-top:8px}.secondary{background:#1b3247;border:1px solid #375970;border-radius:10px;min-height:41px;padding:7px 6px;font-size:12px;font-weight:700}.support{padding:10px 2px 0}.support p{color:var(--muted);font-size:12px;line-height:1.4;margin:0 0 10px}.support-buttons{display:grid;grid-template-columns:1.2fr 1fr 1fr;gap:7px}.history{margin-top:12px;padding:12px;background:var(--panel);border:1px solid var(--border);border-radius:12px}.history-head{display:flex;justify-content:space-between;align-items:center}.history h2{font-size:15px;margin:0}.history-head button{border:0;background:transparent;color:var(--mint)}.history ol{margin:8px 0;padding-left:24px}.history li{margin:8px 0;padding:6px 0;border-bottom:1px solid #2f4a5f;font-size:13px}.history li small{display:block;color:var(--muted);margin-top:5px;font-size:12px}.foot{color:#6c8998;line-height:1.45;font-size:10px;margin:18px 0;text-align:center}.overlay{position:fixed;inset:0;z-index:100;background:#020b16c9;display:flex;align-items:flex-end;justify-content:center}.sheet{position:relative;background:#132b40;border:1px solid #548195;border-radius:20px 20px 0 0;padding:13px 12px calc(13px + var(--safe-bottom));max-width:600px;width:100%;max-height:93dvh;overflow:auto;overscroll-behavior:contain}.sheet-head{display:flex;justify-content:space-between;align-items:center;margin-bottom:12px}.sheet-head strong{font-size:18px}.sheet-head button{width:37px;height:37px;border:1px solid #4a7383;background:#203f52;border-radius:10px}.task-list{display:grid;gap:8px}.task-list button{padding:11px;background:#204056;border:1px solid #416577;border-radius:12px;text-align:left;min-height:60px}.task-list b{display:block}.task-list small{display:block;color:var(--muted);margin-top:4px}.editor{display:flex;flex-direction:column;gap:8px;max-height:96dvh;padding-bottom:calc(11px + var(--safe-bottom))}.editor .sheet-head{flex-shrink:0;margin:0}.editor-segments,.physics-models,.coeff-box{display:flex;gap:7px;flex-shrink:0}.editor-segments button,.physics-models button,.coeff-box button{border:1px solid #46687b;background:#1e4154;border-radius:9px;flex:1;min-width:0;min-height:40px;font-size:12px}.editor-segments .active,.physics-models .active,.coeff-box .active{border-color:var(--mint);color:var(--mint);background:#214c4c}.preview-row{flex:0 0 auto;min-height:70px;max-height:145px;overflow:auto;background:#102337;border:1px solid #426478;border-radius:12px;padding:9px}.preview-row small{color:var(--muted);font-size:11px}.pad-preview{font-size:23px;font-family:Georgia,serif;min-height:30px;line-height:1.2;white-space:pre-wrap;overflow-wrap:anywhere}.pad-preview .caret{border-left:2px solid var(--mint);height:25px;display:inline-block;vertical-align:middle}.pad-error{color:var(--danger);font-size:11px;margin:2px 0}.keypad{display:grid;grid-template-columns:repeat(5,minmax(0,1fr));gap:6px;flex:1 1 auto;min-height:0}.keypad button{min-width:0;min-height:42px;background:#26465b;border:1px solid #53798b;border-radius:11px;font-size:22px;font-weight:700}.keypad button:active,.ops button:active{background:#3f776a}.keypad-foot{display:grid;grid-template-columns:55px 55px 55px 1fr;gap:6px;flex:0 0 auto}.keypad-foot button{min-height:46px;background:#254357;border:1px solid #567d89;border-radius:10px;font-weight:750}.keypad-foot .accept{background:var(--mint);color:#07222c}.pad-hint{display:block;color:var(--muted);font-size:10px;line-height:1.3}.help p{font-size:15px;line-height:1.48}@media(max-height:660px){.app-head{height:52px}.main{padding-top:7px}.workspace{min-height:154px;padding:10px}.equation{min-height:70px}.eq-chip,.coef-chip{min-height:41px}.ops>button{min-height:60px}.ops strong{font-size:26px}.work-actions{margin-top:7px;padding:7px}.sub-actions{margin-top:5px}.support{padding-top:7px}.editor{gap:6px}.keypad{gap:4px}.keypad button{min-height:36px;font-size:19px}.keypad-foot button{min-height:41px}.preview-row{min-height:57px}.pad-hint{display:none}}@media(max-width:340px){.main{padding-left:7px;padding-right:7px}.task-picker{font-size:11px}.ops{gap:4px}.ops>button{min-height:58px}.ops small{font-size:9px}.eq-chip,.coef-chip{padding:5px 6px;min-height:40px;font-size:16px}.work-label{font-size:10px}.eyebrow{font-size:9px}.sub-actions .secondary,.support-buttons .secondary{font-size:11px}}

/* R4 iteration 2: contextual editable equation inside the physical builder */
.phys-live{display:flex;gap:4px;align-items:center;justify-content:center;flex-wrap:wrap;flex:0 0 auto;padding:7px 6px;max-height:108px;overflow:auto;border:1px solid #386678;border-radius:11px;background:#0d2537;font-family:Georgia,serif}.phys-live .builder-chip{background:#234752;border:1px solid #4d8b8b;min-height:36px;padding:4px 5px;font-size:clamp(12px,3.7vw,16px);border-radius:8px;line-height:1.2}.phys-live .builder-chip.active{border-color:#55dfbc;outline:1px solid #55dfbc}.phys-live .builder-sign{color:var(--mint);font-size:20px}.keypad.phys-keypad{grid-template-columns:repeat(4,minmax(0,1fr))}@media(max-height:660px){.phys-live{max-height:83px;padding:4px}.phys-live .builder-chip{min-height:30px}}

.history-controls{display:flex;gap:6px}.history-controls button{font-size:12px;padding:7px;min-height:40px}.history h2{font-size:14px}
/* Final touch target iteration: finger-size controls even at 320×568 */
.head-btn,.sheet-head>button{width:44px;height:44px;min-height:44px}
.task-picker,.secondary,.keypad button,.keypad-foot button,.physics-models button,.phys-live .builder-chip,.eq-chip,.coef-chip{min-height:44px}

/* R5: touch-only independent line verification. */
.sub-actions{grid-template-columns:repeat(4,minmax(0,1fr))}
.manual-button{border-color:var(--mint);color:var(--mint)}
.manual-editor{display:flex;flex-direction:column;gap:8px;max-height:96dvh;overflow:auto}
.manual-editor .sheet-head{margin:0;flex:0 0 auto}
.manual-before{border:1px solid #406275;background:#102338;border-radius:10px;padding:8px 10px;flex:0 0 auto;overflow-wrap:anywhere}
.manual-before small,.manual-value small{display:block;color:var(--muted);font-size:10px;margin-bottom:3px}
#manual-current{font-family:Georgia,serif;font-size:clamp(14px,4.2vw,19px)}
.manual-ops{display:grid;grid-template-columns:repeat(4,minmax(0,1fr));gap:4px;flex-shrink:0}
.manual-ops button,.manual-tabs button{min-width:0;min-height:42px;border:1px solid #48677b;background:#204052;border-radius:9px;padding:6px 3px;font-size:clamp(10px,2.8vw,12px);font-weight:700}
.manual-tabs{display:grid;grid-template-columns:1.3fr 1fr 1fr;gap:5px;flex-shrink:0}
.manual-ops button.active,.manual-tabs button.active{border-color:var(--mint);background:#254a48;color:var(--mint)}
.manual-value{border:1px solid #537483;border-radius:12px;padding:7px 10px;min-height:65px;background:#0f2235;flex:0 0 auto}
#manual-preview{font-family:Georgia,serif;font-size:19px;line-height:1.2;overflow-wrap:anywhere;min-height:26px;max-height:64px;overflow:auto}
.manual-message{font-size:12px;line-height:1.3;color:#bfe4e8;min-height:18px;flex:0 0 auto}
.manual-keypad{display:grid;grid-template-columns:repeat(5,minmax(0,1fr));gap:4px;flex:1 1 auto;min-height:0}
.manual-keypad button{min-width:0;min-height:36px;border:1px solid #477589;background:#24475b;border-radius:9px;font-size:clamp(15px,4.4vw,19px);font-weight:750}
.manual-footer{display:grid;grid-template-columns:56px 46px 46px 1fr;gap:4px;flex:0 0 auto}
.manual-footer button{min-height:44px;border:1px solid #54798a;background:#27485a;border-radius:10px;font-weight:750}
.manual-footer button.accept{background:var(--mint);color:#06292b}
.manual-hint{color:var(--muted);font-size:10px}
@media(max-height:620px){.manual-editor{gap:4px;padding-top:7px}.manual-before{padding:5px 8px}.manual-value{min-height:52px;padding:4px 8px}.manual-keypad button{min-height:32px}.manual-hint{display:none}.manual-tabs button,.manual-ops button{min-height:36px}}
@media(max-width:360px){.sub-actions .secondary{font-size:10px;padding:3px 2px}.manual-ops button{font-size:10px}}

.manual-keypad button.change-page{background:#1c5e56;border-color:#55dfbc;color:#dbfff4}
.manual-keypad{grid-template-rows:repeat(4,minmax(44px,1fr));flex:0 0 auto}
.manual-keypad button{min-height:44px}
@media(max-height:620px){.manual-keypad{grid-template-rows:repeat(4,44px)}.manual-keypad button{min-height:44px}.manual-message{font-size:11px}.manual-before #manual-current{font-size:15px}}

/* R6 signed operand semantics and context-first explicit workflow */
.neutralize{display:block;width:100%;padding:8px 12px;min-height:51px;border-radius:12px;color:#09272a;background:#64dfc0;font-size:clamp(12px,3.2vw,15px);font-weight:800;text-align:center;margin:0 0 6px;}
.neutralize[hidden]{display:none}.action-hint{font-size:11px;color:#c4d9e1;line-height:1.35;margin:0 0 8px}
.operation-sheet{display:flex;flex-direction:column;gap:10px;max-height:96dvh;overflow:auto;padding-bottom:calc(14px + var(--safe-bottom))}
.operation-sheet .sheet-head{margin:0}.operation-types{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:6px}
.operation-types button{background:#244459;border:1px solid #527588;border-radius:10px;min-height:48px;font-size:13px;font-weight:700}
.operation-types button.active,.operation-chips button.active{background:#245e53;color:#d9fff6;border-color:#70edca;outline:1px solid #70edca}
.operation-summary{background:#103831;border:1px solid #63b89f;border-radius:12px;color:#ddfff4;padding:11px 12px;font-size:clamp(13px,3.5vw,16px);min-height:48px;font-weight:750;line-height:1.35}
.operation-section strong{font-size:13px;display:block;margin:0 0 6px;color:#cfe8f1}
.operation-chips{display:flex;flex-wrap:wrap;gap:5px;max-height:102px;overflow:auto}
.operation-chips button{border:1px solid #55798b;border-radius:10px;background:#254459;min-height:45px;padding:6px 10px;font-weight:700;font-size:14px;max-width:100%;overflow-wrap:anywhere}
.operation-number{font:22px Georgia,serif;color:#d0f5ed;background:#0e2335;border:1px solid #466578;padding:7px 12px;min-height:44px;border-radius:10px}
.operation-keypad{display:grid;grid-template-columns:repeat(5,minmax(0,1fr));gap:5px;margin-top:6px}
.operation-keypad button{min-height:45px;border:1px solid #4e7488;border-radius:9px;background:#24475a;font-size:20px;font-weight:750}
.operation-error{font-size:12px;color:#ffd8aa;min-height:0;line-height:1.45}.operation-error:empty{display:none}
.operation-apply{background:#55dfbc;color:#072b2e;min-height:54px;border:0;border-radius:12px;font-size:15px;font-weight:850;width:100%;flex-shrink:0}
.manual-shortcuts{display:flex;flex-wrap:wrap;gap:5px;align-items:center;max-height:105px;overflow:auto;flex:0 0 auto}
.manual-shortcuts small{width:100%;color:#b5d1db;font-size:11px}
.manual-shortcuts button{min-height:40px;border:1px solid #568b95;border-radius:8px;padding:5px 9px;background:#245164;font-size:13px}
.manual-shortcuts .unit-chip{color:#92efce;background:#193b48}
.action-label button{min-width:116px;min-height:44px}
@media(max-height:620px){.operation-sheet{gap:5px}.operation-types button{min-height:44px}.operation-summary{padding:6px 8px;min-height:41px}.operation-chips{max-height:64px}.operation-keypad button{min-height:42px}.operation-section strong{margin-bottom:2px}.operation-number{min-height:39px;padding:5px 10px}.operation-apply{min-height:48px}.manual-shortcuts{max-height:72px}}

.custom-number-toggle{color:#baf5e6;background:#193f4d;border:1px solid #4b827d;border-radius:9px;min-height:44px;font-size:13px;font-weight:750;text-align:left;padding:8px 12px}
.custom-number-panel{border:1px solid #365d67;border-radius:12px;background:#0e2637;padding:6px}
.operation-sheet{gap:7px}
.operation-apply{position:sticky;bottom:0;z-index:2;box-shadow:0 -6px 16px #051725}
@media(max-height:620px){.operation-chips{max-height:87px}.operation-sheet{gap:5px}.operation-sheet .operation-types{gap:4px}.operation-summary{font-size:13px}.operation-section strong{font-size:12px}}

.manual-stage{font-size:12px;font-weight:850;color:#70e8c7;min-height:22px}
.manual-stage ~ .manual-ops[hidden],.manual-stage ~ .manual-tabs[hidden]{display:none}
#manual-keypad[hidden],#manual-shortcuts[hidden],#manual-key-toggle[hidden],#manual-stage-back[hidden]{display:none!important}
.manual-footer{position:sticky;bottom:0;padding:5px 0;background:#132b40;z-index:4}
.manual-editor{gap:6px}.manual-before{padding:5px 9px}.manual-value{min-height:57px;padding:6px 9px}
.manual-message:empty{display:none}
@media(max-height:620px){.manual-editor{gap:5px}.manual-before small{display:none}.manual-before{padding:6px 8px}.manual-value{min-height:50px}.manual-shortcuts{max-height:115px}.manual-keypad{grid-template-rows:repeat(4,42px)}.manual-keypad button{min-height:42px}.manual-footer button{min-height:44px}}

@media(max-height:620px){.manual-editor .manual-shortcuts{max-height:147px}.manual-editor .manual-shortcuts .unit-chip{min-height:44px}}

.manual-stage{display:flex;justify-content:space-between;align-items:center;gap:6px}.manual-stage button{min-height:38px;padding:4px 9px;border:1px solid #517c80;border-radius:8px;background:#234257;font-size:12px;font-weight:750}.manual-editor .sheet-head strong{font-size:clamp(15px,5vw,18px)}
.manual-editor .manual-footer{grid-template-columns:48px 48px 48px minmax(0,1fr);gap:5px}.manual-footer .accept{font-size:clamp(12px,3.4vw,15px);line-height:1.2;padding:3px 6px;overflow-wrap:normal}
@media(max-height:620px){.manual-editor .manual-keypad{grid-template-rows:repeat(4,44px)}.manual-editor .manual-footer{padding:3px 0}.manual-editor .manual-footer button{min-height:44px}}