fix(ui): add ARIA semantics to theme picker buttons (#372)

[FelineFantasy]

Summary

Add ARIA semantics to theme picker buttons for better screen reader accessibility.

Changes:

• Wrap theme options in role="radiogroup" with aria-label="Theme"
• Add role="radio" and aria-checked={selected} to each theme button
• Screen readers now announce which theme is currently active

Closes #372

How to test

pnpm lint && pnpm test && pnpm build

Manual test:

1. Run pnpm dev
2. Go to Dashboard -> Settings -> Appearance
3. Enable screen reader (VoiceOver/NVDA)
4. Navigate through theme options — active theme should be announced as "selected"
5. Click different themes — selection state should update correctly

Code review (Bugbot)

• Not applicable (UI accessibility change, no backend/auth/ingest changes)

Checklist

• Tests pass (pnpm test)
• Lint passes (pnpm lint)
• Build succeeds (pnpm build)
• CHANGELOG.md updated under [Unreleased] (if user-facing)
• Documentation updated (if needed)
• New code follows the existing style
• No breaking changes

Security / privacy

N/A — UI-only change, no data handling implications.