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/components/TopCountriesBarChart.tsx

@@ -13,7 +13,7 @@ import {
   YAxis,
 } from "recharts";
 import type { TooltipContentProps } from "recharts/types/component/Tooltip";
-import { tokens, makeStyles } from "@fluentui/react-components";
+import { tokens, makeStyles, Text } from "@fluentui/react-components";
 import {
   CHART_PALETTE,
   CHART_AXIS_TEXT_TOKEN,
@@ -65,6 +65,16 @@ const useStyles = makeStyles({
     width: "100%",
     overflowX: "hidden",
   },
+  emptyWrapper: {
+    width: "100%",
+    minHeight: `${String(MIN_CHART_HEIGHT)}px`,
+    display: "flex",
+    alignItems: "center",
+    justifyContent: "center",
+  },
+  emptyText: {
+    color: tokens.colorNeutralForeground3,
+  },
 });
 
 // ---------------------------------------------------------------------------
@@ -140,6 +150,15 @@ export function TopCountriesBarChart({
   const styles = useStyles();
 
   const entries = buildEntries(countries, countryNames);
+
+  if (entries.length === 0) {
+    return (
+      <div className={styles.emptyWrapper}>
+        <Text className={styles.emptyText}>No bans in this time range.</Text>
+      </div>
+    );
+  }
+
   const chartHeight = Math.max(entries.length * BAR_HEIGHT_PX, MIN_CHART_HEIGHT);
 
   const primaryColour = resolveFluentToken(CHART_PALETTE[0] ?? "");