feat: Sentry error monitoring for React frontend (#98)

* 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>

* feat: add Sentry error monitoring and tracing to FastAPI backend

- Install sentry-sdk[fastapi] with auto-enabled FastAPI + Anthropic
  integrations
- Init before app = FastAPI() with env-aware sample rates
  (100% dev, 20% prod)
- Filter /health endpoint from traces to reduce noise
- Add SENTRY_DSN to config settings

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/main.tsx

@@ -1,11 +1,18 @@
+import "./instrument"; // Sentry must init before any other imports
+
 import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
+import { reactErrorHandler } from '@sentry/react'
 import { HelmetProvider } from 'react-helmet-async'
 import { Toaster } from 'sonner'
 import './index.css'
 import App from './App'
 
-createRoot(document.getElementById('root')!).render(
+createRoot(document.getElementById('root')!, {
+  onUncaughtError: reactErrorHandler(),
+  onCaughtError: reactErrorHandler(),
+  onRecoverableError: reactErrorHandler(),
+}).render(
   <StrictMode>
     <HelmetProvider>
       {/* Toast notification system - theme syncs automatically via CSS custom properties */}