Add dashboard country charts (Stages 1–3)

- Install Recharts v3 as the project charting library
- Add chartTheme utility with Fluent UI v9 token resolution helper
  and a 5-colour categorical palette (resolves CSS vars at runtime)
- Add TopCountriesPieChart: top-4 + Other slice, Tooltip, Legend
- Add TopCountriesBarChart: horizontal top-20 bar chart
- Add useDashboardCountryData hook (wraps /api/dashboard/bans/by-country)
- Integrate both charts into DashboardPage in a responsive chartsRow
  (side-by-side on wide screens, stacked on narrow)
- All tsc --noEmit and eslint checks pass with zero warnings

frontend/package.json

@@ -19,7 +19,8 @@
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-router-dom": "^6.27.0",
-    "react-simple-maps": "^3.0.0"
+    "react-simple-maps": "^3.0.0",
+    "recharts": "^3.8.0"
   },
   "devDependencies": {
     "@eslint/js": "^9.13.0",