feat: add Sentry error monitoring, tracing, and session replay

- Install @sentry/react and @sentry/vite-plugin
- Create instrument.ts with error monitoring, browser tracing (20% prod),
  and session replay (10% sessions, 100% on errors)
- Wire React 19 reactErrorHandler() on createRoot error hooks
- Wrap router with wrapCreateBrowserRouterV7 for route-aware transactions
- Configure sentryVitePlugin for source map uploads
- Add VITE_SENTRY_DSN to .env.example

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

frontend/.env.example

@@ -1,2 +1,5 @@
 # API URL - defaults to http://localhost:8000 if not set
 VITE_API_URL=http://localhost:8000
+
+# Sentry error monitoring (optional in dev, required in production)
+VITE_SENTRY_DSN=