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/vite.config.ts

@@ -1,11 +1,20 @@
 /// <reference types="vitest/config" />
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
+import { sentryVitePlugin } from '@sentry/vite-plugin'
 import path from 'path'
 
 // https://vite.dev/config/
 export default defineConfig({
-  plugins: [react()],
+  plugins: [
+    react(),
+    sentryVitePlugin({
+      org: process.env.SENTRY_ORG,
+      project: process.env.SENTRY_PROJECT,
+      authToken: process.env.SENTRY_AUTH_TOKEN,
+      silent: !process.env.SENTRY_AUTH_TOKEN, // Don't error in local dev
+    }),
+  ],
   server: {
     host: '0.0.0.0',
     watch: {
@@ -24,6 +33,7 @@ export default defineConfig({
     include: ['src/**/*.{test,spec}.{ts,tsx}'],
   },
   build: {
+    sourcemap: 'hidden', // Generate source maps but don't expose them publicly
     rollupOptions: {
       output: {
         manualChunks: {