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/package.json

@@ -18,6 +18,8 @@
     "@dnd-kit/sortable": "^10.0.0",
     "@dnd-kit/utilities": "^3.2.2",
     "@monaco-editor/react": "^4.7.0",
+    "@sentry/react": "^10.42.0",
+    "@sentry/vite-plugin": "^5.1.1",
     "@stripe/stripe-js": "^8.7.0",
     "@xyflow/react": "^12.10.0",
     "axios": "^1.13.4",