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/instrument.ts

@@ -0,0 +1,25 @@
+import * as Sentry from "@sentry/react";
+
+Sentry.init({
+  dsn: import.meta.env.VITE_SENTRY_DSN,
+  environment: import.meta.env.MODE,
+
+  integrations: [
+    Sentry.browserTracingIntegration(),
+    Sentry.replayIntegration({
+      maskAllText: false,
+      blockAllMedia: false,
+    }),
+  ],
+
+  // Tracing — capture 100% in dev, 20% in production
+  tracesSampleRate: import.meta.env.PROD ? 0.2 : 1.0,
+  tracePropagationTargets: [
+    "localhost",
+    /^https:\/\/api\.resolutionflow\.com/,
+  ],
+
+  // Session Replay — record 10% of sessions, 100% of error sessions
+  replaysSessionSampleRate: 0.1,
+  replaysOnErrorSampleRate: 1.0,
+});