feat: add NotificationBell error-boundary fallback UI with retry action (Closes #833)

[waterWang]

Summary

Adds a compact, attractive error-boundary fallback UI for the NotificationBell component, as specified in issue #833.

Changes

• NotificationBell.tsx: Added NotificationBellErrorFallback component — a compact error-state bell icon with a subtle AlertCircle indicator that shows when the NotificationBell crashes. Clicking it calls resetErrorBoundary to recover.
• NotificationBell.tsx: Added NotificationBellWithErrorBoundary wrapper component that wraps the NotificationBell in the project's ErrorBoundary with the custom fallback.
• NotificationBell.tsx: The default export is now NotificationBellWithErrorBoundary so existing consumers get error protection automatically.
• NotificationBell.error-boundary.test.tsx: 14 focused tests covering the fallback component rendering, accessibility, retry behavior, and the wrapper's integration with the ErrorBoundary.

Design

• Compact bell icon matching the original NotificationBell size (44×44px hit area)
• Red-tinted (text-destructive) error state with AlertCircle indicator
• Tooltip explaining "Notifications unavailable — click to retry"
• Full WCAG 2.1 AA accessibility: semantic button, aria-label, focus-visible rings
• Design-token colors (light + dark mode safe)
• No hardcoded hex values

Testing

• 14 new tests all pass
• All 16 existing NotificationBell tests still pass
• All 5 ErrorRecoveryScreen tests still pass

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