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/hooks/useBanTrend.ts

@@ -23,6 +23,8 @@ export interface UseBanTrendResult {
   isLoading: boolean;
   /** Error message or `null`. */
   error: string | null;
+  /** Re-fetch the data immediately. */
+  reload: () => void;
 }
 
 // ---------------------------------------------------------------------------
@@ -79,5 +81,5 @@ export function useBanTrend(
     };
   }, [load]);
 
-  return { buckets, bucketSize, isLoading, error };
+  return { buckets, bucketSize, isLoading, error, reload: load };
 }

frontend/src/hooks/useDashboardCountryData.ts

@@ -29,6 +29,8 @@ export interface UseDashboardCountryDataResult {
   isLoading: boolean;
   /** Error message or `null`. */
   error: string | null;
+  /** Re-fetch the data immediately. */
+  reload: () => void;
 }
 
 // ---------------------------------------------------------------------------
@@ -91,5 +93,5 @@ export function useDashboardCountryData(
     };
   }, [load]);
 
-  return { countries, countryNames, bans, total, isLoading, error };
+  return { countries, countryNames, bans, total, isLoading, error, reload: load };
 }

frontend/src/hooks/useJailDistribution.ts

@@ -23,6 +23,8 @@ export interface UseJailDistributionResult {
   isLoading: boolean;
   /** Error message or `null`. */
   error: string | null;
+  /** Re-fetch the data immediately. */
+  reload: () => void;
 }
 
 // ---------------------------------------------------------------------------
@@ -81,5 +83,5 @@ export function useJailDistribution(
     };
   }, [load]);
 
-  return { jails, total, isLoading, error };
+  return { jails, total, isLoading, error, reload: load };
 }