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/pages/DashboardPage.tsx

@@ -8,15 +8,13 @@
 
 import { useState } from "react";
 import {
-  MessageBar,
-  MessageBarBody,
-  Spinner,
   Text,
   ToggleButton,
   Toolbar,
   makeStyles,
   tokens,
 } from "@fluentui/react-components";
+import { ChartStateWrapper } from "../components/ChartStateWrapper";
 import { BanTable } from "../components/BanTable";
 import { BanTrendChart } from "../components/BanTrendChart";
 import { JailDistributionChart } from "../components/JailDistributionChart";
@@ -109,7 +107,7 @@ export function DashboardPage(): React.JSX.Element {
   const [timeRange, setTimeRange] = useState<TimeRange>("24h");
   const [originFilter, setOriginFilter] = useState<BanOriginFilter>("all");
 
-  const { countries, countryNames, isLoading: countryLoading, error: countryError } =
+  const { countries, countryNames, isLoading: countryLoading, error: countryError, reload: reloadCountry } =
     useDashboardCountryData(timeRange, originFilter);
 
   return (
@@ -143,27 +141,28 @@ export function DashboardPage(): React.JSX.Element {
           </Text>
         </div>
         <div className={styles.tabContent}>
-          {countryError != null && (
-            <MessageBar intent="error">
-              <MessageBarBody>{countryError}</MessageBarBody>
-            </MessageBar>
-          )}
-          {countryLoading && countryError == null ? (
-            <Spinner label="Loading chart data…" />
-          ) : (
+          <ChartStateWrapper
+            isLoading={countryLoading}
+            error={countryError}
+            onRetry={reloadCountry}
+            isEmpty={!countryLoading && Object.keys(countries).length === 0}
+            emptyMessage="No ban data for the selected period."
+          >
             <div className={styles.chartsRow}>
               <div className={styles.chartCard}>
                 <TopCountriesPieChart
                   countries={countries}
                   countryNames={countryNames}
                 />
-              </div>              <div className={styles.chartCard}>
+              </div>
+              <div className={styles.chartCard}>
                 <TopCountriesBarChart
                   countries={countries}
                   countryNames={countryNames}
                 />
-              </div>            </div>
-          )}
+              </div>
+            </div>
+          </ChartStateWrapper>
         </div>
       </div>