feat: implement Umami page speed tracking via Web Vitals

- Add WebVitalsTracker component using useReportWebVitals
- Report LCP, CLS, FID, FCP, TTFB, and INP as Umami events
- Include rating (good/needs-improvement/poor) for meaningful metrics

components/analytics/AnalyticsShell.tsx

@@ -9,6 +9,9 @@ const DynamicAnalyticsProvider = dynamic(() => import('./AnalyticsProvider'), {
 const DynamicScrollDepthTracker = dynamic(() => import('./ScrollDepthTracker'), {
   ssr: false,
 });
+const DynamicWebVitalsTracker = dynamic(() => import('./WebVitalsTracker'), {
+  ssr: false,
+});
 
 export default function AnalyticsShell() {
   const [shouldLoad, setShouldLoad] = useState(false);
@@ -34,6 +37,7 @@ export default function AnalyticsShell() {
     <Suspense fallback={null}>
       <DynamicAnalyticsProvider />
       <DynamicScrollDepthTracker />
+      <DynamicWebVitalsTracker />
     </Suspense>
   );
 }