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/src/setupTests.ts

@@ -0,0 +1,33 @@
+import { expect, afterEach } from "vitest";
+import { cleanup } from "@testing-library/react";
+import * as jestDomMatchers from "@testing-library/jest-dom/matchers";
+
+// Extend Vitest's expect with jest-dom matchers (toBeInTheDocument, etc.)
+expect.extend(jestDomMatchers);
+
+// Ensure React Testing Library cleans up after every test
+afterEach(cleanup);
+
+// Recharts and Fluent UI rely on ResizeObserver which jsdom does not provide.
+class ResizeObserverStub {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+
+globalThis.ResizeObserver = ResizeObserverStub;
+
+// Fluent UI animations rely on matchMedia.
+Object.defineProperty(window, "matchMedia", {
+  writable: true,
+  value: (query: string) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: () => {},
+    removeListener: () => {},
+    addEventListener: () => {},
+    removeEventListener: () => {},
+    dispatchEvent: () => false,
+  }),
+});