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/JailDistributionChart.tsx

@@ -17,10 +17,6 @@ import {
 } from "recharts";
 import type { TooltipContentProps } from "recharts/types/component/Tooltip";
 import {
-  MessageBar,
-  MessageBarBody,
-  Spinner,
-  Text,
   tokens,
   makeStyles,
 } from "@fluentui/react-components";
@@ -30,6 +26,7 @@ import {
   CHART_PALETTE,
   resolveFluentToken,
 } from "../utils/chartTheme";
+import { ChartStateWrapper } from "./ChartStateWrapper";
 import { useJailDistribution } from "../hooks/useJailDistribution";
 import type { BanOriginFilter, TimeRange } from "../types/ban";
 
@@ -77,16 +74,6 @@ const useStyles = makeStyles({
     width: "100%",
     overflowX: "hidden",
   },
-  stateWrapper: {
-    width: "100%",
-    minHeight: `${String(MIN_CHART_HEIGHT)}px`,
-    display: "flex",
-    alignItems: "center",
-    justifyContent: "center",
-  },
-  emptyText: {
-    color: tokens.colorNeutralForeground3,
-  },
 });
 
 // ---------------------------------------------------------------------------
@@ -153,66 +140,50 @@ export function JailDistributionChart({
   origin,
 }: JailDistributionChartProps): React.JSX.Element {
   const styles = useStyles();
-  const { jails, isLoading, error } = useJailDistribution(timeRange, origin);
-
-  if (error != null) {
-    return (
-      <MessageBar intent="error">
-        <MessageBarBody>{error}</MessageBarBody>
-      </MessageBar>
-    );
-  }
-
-  if (isLoading) {
-    return (
-      <div className={styles.stateWrapper}>
-        <Spinner label="Loading chart data…" />
-      </div>
-    );
-  }
-
-  if (jails.length === 0) {
-    return (
-      <div className={styles.stateWrapper}>
-        <Text className={styles.emptyText}>No ban data for the selected period.</Text>
-      </div>
-    );
-  }
+  const { jails, isLoading, error, reload } = useJailDistribution(timeRange, origin);
 
   const entries = buildEntries(jails);
   const chartHeight = Math.max(entries.length * BAR_HEIGHT_PX, MIN_CHART_HEIGHT);
-
   const primaryColour = resolveFluentToken(CHART_PALETTE[0] ?? "");
   const axisColour = resolveFluentToken(CHART_AXIS_TEXT_TOKEN);
   const gridColour = resolveFluentToken(CHART_GRID_LINE_TOKEN);
 
   return (
-    <div className={styles.wrapper} style={{ height: chartHeight }}>
-      <ResponsiveContainer width="100%" height="100%">
-        <BarChart
-          layout="vertical"
-          data={entries}
-          margin={{ top: 4, right: 16, bottom: 4, left: 8 }}
-        >
-          <CartesianGrid strokeDasharray="3 3" stroke={gridColour} horizontal={false} />
-          <XAxis
-            type="number"
-            tick={{ fill: axisColour, fontSize: 12 }}
-            axisLine={{ stroke: gridColour }}
-            tickLine={false}
-          />
-          <YAxis
-            type="category"
-            dataKey="name"
-            width={160}
-            tick={{ fill: axisColour, fontSize: 12 }}
-            axisLine={false}
-            tickLine={false}
-          />
-          <Tooltip content={JailTooltip} cursor={{ fill: "transparent" }} />
-          <Bar dataKey="value" fill={primaryColour} radius={[0, 3, 3, 0]} />
-        </BarChart>
-      </ResponsiveContainer>
-    </div>
+    <ChartStateWrapper
+      isLoading={isLoading}
+      error={error}
+      onRetry={reload}
+      isEmpty={jails.length === 0}
+      emptyMessage="No bans in this time range."
+      minHeight={MIN_CHART_HEIGHT}
+    >
+      <div className={styles.wrapper} style={{ height: chartHeight }}>
+        <ResponsiveContainer width="100%" height="100%">
+          <BarChart
+            layout="vertical"
+            data={entries}
+            margin={{ top: 4, right: 16, bottom: 4, left: 8 }}
+          >
+            <CartesianGrid strokeDasharray="3 3" stroke={gridColour} horizontal={false} />
+            <XAxis
+              type="number"
+              tick={{ fill: axisColour, fontSize: 12 }}
+              axisLine={{ stroke: gridColour }}
+              tickLine={false}
+            />
+            <YAxis
+              type="category"
+              dataKey="name"
+              width={160}
+              tick={{ fill: axisColour, fontSize: 12 }}
+              axisLine={false}
+              tickLine={false}
+            />
+            <Tooltip content={JailTooltip} cursor={{ fill: "transparent" }} />
+            <Bar dataKey="value" fill={primaryColour} radius={[0, 3, 3, 0]} />
+          </BarChart>
+        </ResponsiveContainer>
+      </div>
+    </ChartStateWrapper>
   );
 }