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.

lib/api/stats.ts

@@ -22,9 +22,10 @@ export interface ScreenResolutionStat {
 }
 
 export interface PerformanceStats {
-  lcp: number
-  cls: number
-  inp: number
+  lcp: number | null
+  cls: number | null
+  inp: number | null
+  samples: number
 }
 
 export interface PerformanceByPageStat {