a11y(toast): announce updates politely

[Devadakene]

Closes #557

Description

This PR introduces a debounced live-region announcement to improve accessibility when toast content updates. Previously, screen-reader users did not receive feedback on the overall count and status of visible toasts when a new toast arrived. This adds a polite live-region announcement that dynamically states the overall count and specific variants (success/error), using a 500ms debounce to prevent spamming the screen reader queue during rapid updates.

Closes #

Type of Change

• Bug fix (non-breaking change that resolves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (no functional change, internal code improvement)
• Documentation update

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Pre-flight Checklist

All items must be checked before requesting review.

• npm run lint passes with no errors
• npm test passes with no failures
• npm run build completes successfully

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Testing & Coverage

• Module test coverage for impacted files meets or exceeds the 95% minimum threshold
  (run npm test -- --coverage and check the per-file table)
• If this PR adds or modifies UI components, accessibility tests were written using
  the shared utilities in src/test-utils/a11y.tsx

What was tested?

• Debounce logic: Verified that rapid, successive toast creations (within the 500ms window) result in a single, consolidated announcement.
• Zero notifications edge-case: Verified that when all toasts are dismissed, the announcement dynamically resets to report "0 notifications".
• String correctness: Verified that the live region correctly announces counts for specific variants (e.g., "2 notifications (1 error, 1 success)").

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Accessibility & Security Notes

Accessibility

Yes, added a new polite live-region announcement specifically targeted at screen readers to communicate changes to the toast stack that were previously omitted. Testing covers assertions on aria-live regions and proper timing to ensure assistive technologies have adequate time to announce the updates.

Security

N/A – no authentication or sensitive user logic was altered. Ensure statusAnnouncement renders plainly, which it safely does via React state string interpolation.

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