feat(frontend): add route loading skeleton and error boundary

[chriswritescode-dev]

Summary

• Add route loading skeleton (AppLoadingSkeleton) for full-page transitions with sidebar, content grid, and mobile tab bar
• Add route error boundary (RouteErrorBoundary) with network-aware detection and auto-reconnect
• Wrap auth loaders in try/catch with graceful network error handling and default fallback config

Changes

• frontend/src/App.tsx — wires up useNavigation() loading state, errorElement, and HydrateFallback
• frontend/src/components/ui/AppLoadingSkeleton.tsx (new) — full-page skeleton component
• frontend/src/components/ui/RouteErrorBoundary.tsx (new) — error boundary with offline detection
• frontend/src/lib/auth-loaders.ts — wraps loaders in try/catch with graceful network error handling

Checks

• pnpm lint
• pnpm build