fix: dashboard flickering on poll, animation replay, and scroll

- Add placeholderData to useRouteMetrics and usePunchcard hooks so data
  stays stable between refetches instead of going undefined → flicker
- Disable Recharts animation on Treemap (isAnimationActive=false)
- Make .content scrollable (overflow-y: auto, flex: 1, min-height: 0)
  so charts below the fold are accessible

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/api/queries/dashboard.ts

@@ -106,7 +106,7 @@ export function usePunchcard(application?: string) {
   return useQuery({
     queryKey: ['dashboard', 'punchcard', application],
     queryFn: () => fetchJson<PunchcardCell[]>('/search/stats/punchcard', { application }),
-    placeholderData: (prev: PunchcardCell[] | undefined) => prev,
+    placeholderData: (prev: PunchcardCell[] | undefined) => prev ?? [],
     refetchInterval,
   });
 }