feat: migrate Treemap and PunchcardHeatmap to Recharts

Replace custom SVG chart implementations with Recharts components:
- Treemap: uses Recharts Treemap with custom content renderer for
  SLA-colored cells, labels, and click navigation
- PunchcardHeatmap: uses Recharts ScatterChart with custom Rectangle
  shape for weekday x hour heatmap grid cells

Both use ResponsiveContainer (no more explicit width/height props) and
rechartsTheme from the design system for consistent tooltip styling.

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

ui/package.json

@@ -21,6 +21,7 @@
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
     "react-router": "^7.13.1",
+    "recharts": "^3.8.1",
     "swagger-ui-dist": "^5.32.0",
     "zustand": "^5.0.11"
   },