feat: add Core Web Vitals reporting to PostHog

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/main.tsx

@@ -6,6 +6,7 @@ import { createRoot } from 'react-dom/client'
 import { reactErrorHandler } from '@sentry/react'
 import { HelmetProvider } from 'react-helmet-async'
 import { PostHogProvider } from '@posthog/react'
+import { initWebVitals } from './lib/webVitals'
 import { Toaster } from 'sonner'
 import './index.css'
 import App from './App'
@@ -22,6 +23,9 @@ if (posthogKey) {
   })
 }
 
+// Start Web Vitals reporting to PostHog
+initWebVitals()
+
 createRoot(document.getElementById('root')!, {
   onUncaughtError: reactErrorHandler(),
   onCaughtError: reactErrorHandler(),