Add BanTrendChart component and useBanTrend hook

- Add BanTrendBucket / BanTrendResponse interfaces to types/ban.ts
- Add dashboardBansTrend endpoint constant to api/endpoints.ts
- Add fetchBanTrend() to api/dashboard.ts
- Create useBanTrend hook with abort-safe data fetching
- Create BanTrendChart: AreaChart with gradient fill, dynamic
  X-axis labels per range, custom tooltip, loading/error/empty states
- tsc --noEmit and ESLint pass with zero warnings

frontend/src/api/endpoints.ts

@@ -30,6 +30,7 @@ export const ENDPOINTS = {
   dashboardStatus: "/dashboard/status",
   dashboardBans: "/dashboard/bans",
   dashboardBansByCountry: "/dashboard/bans/by-country",
+  dashboardBansTrend: "/dashboard/bans/trend",
 
   // -------------------------------------------------------------------------
   // Jails