[FE-05] Fix silent 401 handling and add global error boundary for API failures

[Leothosine]

Problem

Three error-handling failures exist:

1. frontend/lib/api/client.ts:24-27 has 401 redirect logic commented out. Expired tokens silently fail with no redirect to login.
2. QueryProvider retries with retry: 2 and no exponential backoff - during an outage, three rapid retries hammer the backend simultaneously.
3. No React error boundary wraps the app. API errors that propagate to render phase cause an unrecoverable white screen.

Proposed Solution

1. Uncomment and implement the 401 interceptor in client.ts: clear auth state from Zustand, clear the cookie, call router.push('/login')
2. Add exponential backoff with jitter to QueryProvider's retry config: retryDelay: (attempt) => Math.min(1000 * 2 ** attempt + Math.random() * 500, 10000)
3. Create frontend/components/GlobalErrorBoundary.tsx (class component) that catches render errors and shows a friendly fallback with a retry button
4. Wrap children in app/layout.tsx with <GlobalErrorBoundary>

Acceptance Criteria

• An expired token triggers a redirect to /login within one render cycle
• Three failed API retries show an error UI - not a white screen
• Retry button in the error boundary clears the error state and re-renders children
• Network error shows a Connection error toast
• Backoff verified: second retry fires ~2s after first, not immediately

[Menjay7]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. Thank you!