chore: Revert recharts to stable v2

- recharts: 3.7.0 (v3, breaking changes) -> 2.15.4 (stable v2)

Recharts v3 has breaking changes that require code updates.
Sticking with latest v2 for stability.

frontend/package.json

@@ -12,7 +12,7 @@
     "next-auth": "^5.0.0-beta.25",
     "react": "^19.1.0",
     "react-dom": "^19.1.0",
-    "recharts": "^3.7.0",
+    "recharts": "^2.15.4",
     "lucide-react": "^0.574.0",
     "date-fns": "^4.1.0",
     "@radix-ui/react-dialog": "^1.1.15",