Add keyboard and screen-reader support for dashboard widgets.

[Honey-pg]

Summary

Introduce a shared focusable widget shell with live region summaries for pilot metric widgets so users can tab to cards and hear title plus key stats.

Adds keyboard and screen-reader support to dashboard widgets via a shared DashboardWidgetShell and DashboardWidgetA11yContext. All widgets are now tab-reachable with visible focus rings and region labels; four pilot widgets (Streak, Goals, Language Breakdown, PR Metrics) announce dynamic metric summaries through polite live regions.

Closes #2435

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Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that changes existing behavior)
• 📝 Documentation update
• ♻️ Refactor / code cleanup (no functional change)
• ⚡ Performance improvement
• 🔒 Security fix
• 🧪 Tests only

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What Changed

• Added src/components/dashboard/DashboardWidgetA11yContext.tsx — provider + useDashboardWidgetA11y() hook for setSummary / setIsUpdating
• Added src/components/dashboard/DashboardWidgetShell.tsx — focusable role="region" wrapper with sr-only title, live summary region, focus ring, and Enter/Space to focus first inner control
• Updated SortableDashboardWidget.tsx — wraps widget content in DashboardWidgetShell (non-editing mode is tabbable; edit mode uses tabIndex={-1})
• Updated CustomizableDashboard.tsx — wraps the widget grid in DashboardWidgetA11yProvider
• Updated pilot widgets with dynamic summaries:
  • StreakTracker.tsx — current/longest streak
  • GoalTracker.tsx — active vs completed goals
  • LanguageBreakdown.tsx — top language + count
  • PRMetrics.tsx — open PRs + merged (30d)
• Added unit tests: test/components/DashboardWidgetShell.test.tsx, test/components/DashboardWidgetA11yContext.test.tsx

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How to Test

1. Open /dashboard and press Tab repeatedly — each visible widget card should show a focus ring as it receives focus.
2. Tab to the Streak Tracker card and press Enter or Space — focus should move to the first inner button (e.g. Copy/Share).
3. With a screen reader (e.g. VoiceOver), tab to pilot widgets after data loads — you should hear the widget title plus the metric summary.
4. Enable layout edit mode — widget cards should no longer be tabbable before drag/hide controls; drag and hide buttons should still work.
5. Click inner widget buttons with the mouse — behavior should be unchanged.

Expected result: Widgets are keyboard-reachable with visible focus, titled regions for screen readers, and live announcements for the four pilot metrics. No mouse interaction regressions.

Automated checks:

npm run type-check
npm run lint
npm test -- test/components/DashboardWidgetShell.test.tsx test/components/DashboardWidgetA11yContext.test.tsx

[github-actions]

GSSoC Label Checklist 🏷️

@Umbrella-io — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus