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/pages/DashboardTab/DashboardTab.module.css

@@ -2,6 +2,10 @@
   display: flex;
   flex-direction: column;
   gap: 20px;
+  flex: 1;
+  min-height: 0;
+  overflow-y: auto;
+  padding-bottom: 20px;
 }
 
 .refreshIndicator {