feat: move performance to dedicated tab, fix 0/99999 metrics bug

Performance metrics moved from dashboard into a new Performance tab.
Fixed null handling so "No data" shows instead of misleading zeros.
Script no longer sends INP=0 when no interaction occurred.

components/dashboard/SiteNav.tsx

@@ -18,6 +18,7 @@ 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` },