a11y: WCAG AA contrast + ARIA roles + focus management across all pages

- css/ls.css: --text-3 #8898AA → #56687A (5.1:1 contrast), min-height 44px on .btn-primary/.btn-ghost/.sb-link, new .icon-btn utility (44×44px)
- js/api.js: lsConfirm — role=dialog, aria-modal, aria-labelledby, Tab focus trap, restore focus on close; lsToast — aria-live=polite on container, role=alert on errors; live quiz — role=dialog, role=radiogroup, role=radio, aria-checked, keyboard support
- test-run.html: q-opt divs — role=radio/checkbox, aria-checked, tabindex, keyboard enter/space; confirm modal — role=dialog, aria-modal; btn-flag — aria-pressed; dots — aria-label, aria-current; touch targets 44px
- board.html: btn-del-ann — aria-label; reaction buttons — aria-label, aria-pressed
- All 18 HTML files: replace hardcoded color:#8898AA with color:var(--text-3)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/parent.html

@@ -10,7 +10,7 @@
     *, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
     :root {
       --violet: #9B5DE5; --cyan: #06D6E0; --bg: #F8F9FB; --text: #0F172A;
-      --text-2: #475569; --text-3: #8898AA; --border: #E8EBF0;
+      --text-2: #475569; --text-3: #56687A; --border: #E8EBF0;
       --card: #fff; --shadow: 0 2px 16px rgba(15,23,42,0.06);
       --green: #22c55e; --amber: #f59e0b; --red: #ef4444; --blue: #3b82f6;
     }