Resolve accessibility lint suppressions from lint cleanup

[Uarmagan]

What to build

Replace the a11y lint suppressions added during lint cleanup with real, tested accessibility fixes where practical, while preserving the current calendar, task, menu, and date-picker interactions.

This is a single AFK follow-up slice. It covers the current suppressed a11y spots around:

• calendar event and all-day event drag/resize blocks
• invisible resize handles
• day agenda scroll container behavior
• task checkbox behavior
• shortcut hover hint behavior
• context menu wrappers
• overlay panel dismissal and modal semantics
• date picker click-containment wrappers

Acceptance criteria

• Each current biome-ignore lint/a11y/* added during lint cleanup is either removed through a real fix or narrowed with a verified explanation for why the suppression is still necessary.
• Calendar event drag, resize, and right-click behavior still works for timed and all-day events.
• Day agenda keyboard shortcuts and scrolling still work.
• Task checkbox, context menu, overlay panel, shortcut hint, and date picker interactions still work.
• Relevant React Testing Library tests are added or updated for keyboard and screen-reader-facing behavior.
• bun run lint, bun run type-check, and focused web tests pass.

Blocked by

None - can start immediately