feat(stats): add loading, error, and empty states for stats page

[mysterio123865]

Summary

Implements Issue by adding comprehensive loading, error, and empty states to the Stats page, improving user feedback, accessibility, and retry behavior.

Changes

• Added Loading state:

  • Displays a spinner while the initial stats request is in progress

• Added Error state:

  • Shows an EmptyState with a keyboard-accessible Retry action when the initial load fails
  • Preserves existing stats during polling failures and displays an ErrorMessage with a Try again action

• Added Empty state:

  • Displays an EmptyState with a Refresh action when the response contains no usable stats fields

• Improved accessibility:

  • Made loading, error, and empty states mutually exclusive
  • Added aria-live="polite" announcements following the existing admin page pattern

Validation

✅ Stats tests passed (25/25)
✅ 100% coverage for page.tsx
✅ npm run build completed successfully

Note: Repository-wide lint and the full test suite still contain pre-existing failures (api-keys tests and unrelated lint issues in other files). These are unrelated to this change.

Tracking

Closes #385