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

Docs/Tasks.md

@@ -289,7 +289,15 @@ class BanTrendResponse(BaseModel):
 
 ### Task 4.2 — Create the `BanTrendChart` component
 
-**Status:** `not started`
+**Status:** `done`
+
+Created `frontend/src/components/BanTrendChart.tsx` — an area chart using Recharts
+`AreaChart` with a gradient fill, human-readable X-axis time labels (format varies by
+time range), and a custom tooltip. Added `BanTrendBucket`/`BanTrendResponse` types to
+`types/ban.ts`, `dashboardBansTrend` constant to `api/endpoints.ts`, `fetchBanTrend()`
+to `api/dashboard.ts`, and the `useBanTrend` hook at `hooks/useBanTrend.ts`. Component
+handles loading (Spinner), error (MessageBar), and empty states inline.
+`tsc --noEmit` and ESLint pass with zero warnings.
 
 Create `frontend/src/components/BanTrendChart.tsx`. This component renders an **area/line chart** showing the number of bans over time.