feat(web): UX polish and accessibility hardening (US-005)

[Hectormalvarez]

Summary

US-005 per docs/stories/US-005-ux-polish-accessibility.md (absorbs backlog US-003 items 1-3):

• Visible labeled fields in the alarm panel (placeholder-only before); themed placeholder token (4.71:1 AA)
• Semantic WCAG-AA design tokens replacing the per-element opacity soup (error text #f00 4.35:1 -> #ff6b6b 6.27:1)
• Monochrome inline-SVG icons replacing the red alarm / blue zzz / stopwatch emojis; buttons inherit the page font
• Shared control scale across alarm + timer panels; timer steppers always visible (were hover-only, unusable on touch)
• Focus management: focus into panels on open, restored to toggles on close; ring overlay is aria-modal with a Tab trap; outside click cannot close the panel while ringing
• prefers-reduced-motion support (CSS kill switch + reduced-motion-aware shared FLIP helper in shared/dom/flip.ts with unit tests)
• Pure cores untouched: all 135 pre-existing behavior tests pass unchanged; 5 new FLIP tests (140 total)

Verification

• 20/20 Playwright a11y assertions (aria-expanded, focus move/restore/trap, labels, reduced-motion)
• Computed contrast table in the story: all pairs AA (4.71:1 - 12.68:1)
• 14 before/after screenshots: docs/stories/US-005/screenshots/
• make check green; prettier clean