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/Treemap.tsx

@@ -112,6 +112,7 @@ export function Treemap({ items, onItemClick }: TreemapProps) {
         nameKey="name"
         stroke="none"
         content={renderContent}
+        isAnimationActive={false}
       >
         <Tooltip
           contentStyle={rechartsTheme.tooltip.contentStyle}