feat: maximize Sentry free plan coverage for frontend and backend

- ErrorBoundary: use Sentry.ErrorBoundary with crash feedback dialog
- RouteError: capture route errors in Sentry (skip chunk load errors)
- User context: set Sentry user on login (frontend + backend)
- Backend: enable profiling (profiles_sample_rate)
- Frontend: add feedback integration, lower replay rate to conserve quota
- Add temporary verification message for production validation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -17,6 +17,8 @@ if settings.SENTRY_DSN:
         environment="development" if settings.DEBUG else "production",
         send_default_pii=True,
         traces_sample_rate=1.0 if settings.DEBUG else 0.2,
+        # Profiling — included in free plan
+        profiles_sample_rate=1.0 if settings.DEBUG else 0.2,
         # Filter out noisy health check transactions
         traces_sampler=lambda ctx: (
             0.0 if ctx.get("transaction_context", {}).get("name", "").startswith("GET /health") else None