Polish dashboard charts and add frontend tests (Stage 6)

Task 6.1 - Consistent loading/error/empty states across all charts:
- Add ChartStateWrapper shared component with Spinner, error MessageBar
  + Retry button, and friendly empty message
- Expose reload() in useBanTrend, useJailDistribution,
  useDashboardCountryData hooks
- Update BanTrendChart and JailDistributionChart to use ChartStateWrapper
- Add empty state to TopCountriesBarChart and TopCountriesPieChart
- Replace manual loading/error logic in DashboardPage with ChartStateWrapper

Task 6.2 - Frontend tests (5 files, 20 tests):
- Install Vitest v4, jsdom, @testing-library/react, @testing-library/jest-dom
- Add vitest.config.ts (separate from vite.config.ts to avoid Vite v5/v7 clash)
- Add src/setupTests.ts with jest-dom matchers and ResizeObserver/matchMedia stubs
- Tests: ChartStateWrapper (7), BanTrendChart (4), JailDistributionChart (4),
  TopCountriesPieChart (2), TopCountriesBarChart (3)

Task 6.3 - Full QA:
- ruff: clean
- mypy --strict: 52 files, no issues
- pytest: 497 passed
- tsc --noEmit: clean
- eslint: clean (added test-file override for explicit-function-return-type)
- vite build: success

frontend/package.json

@@ -10,7 +10,9 @@
     "preview": "vite preview",
     "lint": "eslint . --ext ts,tsx --report-unused-disable-directives --max-warnings 0",
     "typecheck": "tsc --noEmit",
-    "format": "prettier --write 'src/**/*.{ts,tsx,css}'"
+    "format": "prettier --write 'src/**/*.{ts,tsx,css}'",
+    "test": "vitest run --config vitest.config.ts",
+    "coverage": "vitest run --config vitest.config.ts --coverage"
   },
   "dependencies": {
     "@fluentui/react-components": "^9.55.0",
@@ -24,19 +26,25 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.13.0",
+    "@testing-library/jest-dom": "^6.9.1",
+    "@testing-library/react": "^16.3.2",
+    "@testing-library/user-event": "^14.6.1",
     "@types/node": "^25.3.2",
     "@types/react": "^18.3.12",
     "@types/react-dom": "^18.3.1",
     "@typescript-eslint/eslint-plugin": "^8.13.0",
     "@typescript-eslint/parser": "^8.13.0",
     "@vitejs/plugin-react": "^4.3.3",
+    "@vitest/coverage-v8": "^4.0.18",
     "eslint": "^9.13.0",
     "eslint-config-prettier": "^9.1.0",
     "eslint-plugin-react-hooks": "^5.0.0",
     "jiti": "^2.6.1",
+    "jsdom": "^28.1.0",
     "prettier": "^3.3.3",
     "typescript": "^5.6.3",
     "typescript-eslint": "^8.56.1",
-    "vite": "^5.4.11"
+    "vite": "^5.4.11",
+    "vitest": "^4.0.18"
   }
 }