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/types/ban.ts

@@ -82,3 +82,31 @@ export interface DashboardBanListResponse {
   /** Maximum items per page. */
   page_size: number;
 }
+
+// ---------------------------------------------------------------------------
+// Ban trend
+// ---------------------------------------------------------------------------
+
+/**
+ * A single time bucket in the ban trend series.
+ *
+ * Mirrors `BanTrendBucket` from `backend/app/models/ban.py`.
+ */
+export interface BanTrendBucket {
+  /** ISO 8601 UTC start of the bucket. */
+  timestamp: string;
+  /** Number of bans that started in this bucket. */
+  count: number;
+}
+
+/**
+ * Response from `GET /api/dashboard/bans/trend`.
+ *
+ * Mirrors `BanTrendResponse` from `backend/app/models/ban.py`.
+ */
+export interface BanTrendResponse {
+  /** Time-ordered list of ban-count buckets covering the full window. */
+  buckets: BanTrendBucket[];
+  /** Human-readable bucket size label, e.g. `"1h"`, `"6h"`, `"1d"`, `"7d"`. */
+  bucket_size: string;
+}