a11y: Tasks 7-12 -- Dashboard, Settings, StatsView, BreakScreen, Celebration

- Dashboard: text-text-sec tokens, nav landmark, toast hover persistence,
  goal progressbar ARIA, pomodoro sr-only text
- Settings: h3→h2 heading hierarchy, section aria-labelledby with ids,
  Working Hours heading added
- StatsView: h3→h2, tablist/tab/tabpanel ARIA pattern, sr-only data tables
  for 30-day chart and heatmap, contrast tokens
- BreakScreen: strict-mode focus safety span, breathing phase-only
  announcements, contrast tokens
- Celebration: JS-controlled hover/focus persistence, dismiss buttons,
  Escape key, removed pointer-events:none
- Titlebar: removed redundant role="banner" on <header>

src/lib/components/Titlebar.svelte

@@ -6,7 +6,6 @@
 
 <!-- Invisible drag region – traffic lights on the right -->
 <header
-  role="banner"
   data-tauri-drag-region
   class="group absolute top-0 left-0 right-0 z-50 flex h-10 items-center justify-end pr-3.5 select-none"
 >