Feat/aria live sync announcement

[mrunalC27]

Summary

Added an accessible aria-live="polite" region in DashboardHeader to announce sync status updates for screen reader users after metrics refresh actions.

Closes #216

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

Changes Made

• Added a visually hidden live region using aria-live="polite" and role="status" in DashboardHeader
• Updated announcement text after sync success and failure responses
• Added timeout cleanup logic to clear announcements after 3 seconds
• Prevented stale timeout issues on component unmount

How to Test

Steps for the reviewer to verify this works:

1. Open the dashboard page

2. Trigger the Sync button

3. Verify that screen readers announce:

   • "Metrics refreshed successfully" on success
   • "Sync failed. Please try again." on failure

4. Confirm the announcement clears automatically after 3 seconds

Screenshots (after UI change)

Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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