Client #52: Global error boundary and 404 page

[FadyGergesRezk]

Why

Implement error handling and 404 routing for the app. Users need a graceful fallback when the app crashes, and a clear 404 page when they hit non-existent routes.

What changed

• ErrorBoundary.tsx (new) — React class component that catches unhandled rendering errors inside the authenticated app. Renders a centered ErrorCard with "Go home" and "Try again" (remounts via resetKey) actions.
• NotFoundPage.tsx (new) — Functional component for the catch-all route. Renders a 404 heading with "Go back" and "Go home" buttons.
• ErrorCard.tsx (new) — Shared full-page error UI used by both GlobalErrorBoundary and AuthenticatedApp, so there is one visual design for all error states.
• main.tsx — ThemeProvider wraps everything so all error states (auth and runtime) are themed. AuthenticatedApp receives the QueryClient instance.
• AuthenticatedApp.tsx — Auth errors render via ErrorCard directly. On success, mounts QueryClientProvider > GlobalErrorBoundary > App, so the boundary only covers the authenticated subtree and cannot intercept auth errors.
• routes.tsx — Added catch-all route: { path: '*', element: <NotFoundPage /> } as the last child of the AppShell route.

Notes

GlobalErrorBoundary intentionally sits inside AuthenticatedApp (not outside it) because auth failures are async and cannot be caught by an error boundary — they must be handled with useState + try/catch. The boundary covers only runtime rendering errors after auth succeeds.

Testing

• pnpm test — 30/30 passing
• pnpm build — clean
• pnpm typecheck — clean

Closes #52

[raphael-frank]

lgtm! reasonable changes, merging.