refactor: remove performance insights (Web Vitals) feature entirely

Remove Performance tab, PerformanceStats component, settings toggle,
Web Vitals observers from tracking script, and all related API types
and SWR hooks. Duration tracking is preserved.

components/dashboard/SiteNav.tsx

@@ -18,7 +18,6 @@ export default function SiteNav({ siteId }: SiteNavProps) {
 
   const tabs = [
     { label: 'Dashboard', href: `/sites/${siteId}` },
-    { label: 'Performance', href: `/sites/${siteId}/performance` },
     { label: 'Journeys', href: `/sites/${siteId}/journeys` },
     { label: 'Funnels', href: `/sites/${siteId}/funnels` },
     { label: 'Behavior', href: `/sites/${siteId}/behavior` },