Add Web Vitals performance tracking (LCP, INP, CLS, FCP, TTFB)

End-to-end performance metrics tracking: dedicated database table with
percentile aggregation, tracker collection behind data-perf attribute,
/api/send handling, report API endpoints, and Performance dashboard page
with threshold-based metric cards, time-series chart, and per-page breakdown.

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

src/components/hooks/useWebsiteNavItems.tsx

@@ -8,7 +8,7 @@ import {
   User,
   UserPlus,
 } from '@/components/icons';
-import { Funnel, Lightning, Magnet, Money, Network, Path, Target } from '@/components/svg';
+import { Funnel, Gauge, Lightning, Magnet, Money, Network, Path, Target } from '@/components/svg';
 import { useMessages } from './useMessages';
 import { useNavigation } from './useNavigation';
 
@@ -53,6 +53,12 @@ export function useWebsiteNavItems(websiteId: string) {
           icon: <Clock />,
           path: renderPath('/realtime'),
         },
+        {
+          id: 'performance',
+          label: t(labels.performance),
+          icon: <Gauge />,
+          path: renderPath('/performance'),
+        },
         {
           id: 'compare',
           label: t(labels.compare),