feat(jellyseer): stat widgets + rich Requests tab (slice 3/3)

Frontend for Jellyseerr request stats, reusing the generic stat abstraction.

- api/jellyseerr.ts + hooks/useJellyseer.ts: fetchJellyseerrStats +
  useJellyseerrStats (polls /api/jellyseerr/stats, no-retry; shares the backend
  cache with the widgets).
- Two reusable widgets backed by the stat/stats_overview kinds:
  - RequestStatWidget: a single selected stat (big value + label).
  - RequestsOverviewWidget: a MetricCard grid of all stats + a recent-requests
    list with status/media-status badges.
- registry.ts: Jellyfin gains `stat` (a dropdown over
  total/pending/approved/declined/processing/available — the "extract one stat
  into a widget" affordance, rendered as a Select via the existing enum UI) and
  `stats_overview` widget kinds, wired to the new components.
- RequestsTab rewritten: live stats grid (6 counts) + recent-requests list +
  a hint to pin individual stats via the Request stat widget. Reads
  jellyseerr_url from config and the now-secret jellyseerr_api_key from
  secrets_set.

Tests: RequestStatWidget + RequestsOverviewWidget rendering/error; RequestsTab
not-configured CTA, configured stats grid, and error states. 184/184 frontend
tests pass; tsc + ESLint clean.

frontend/src/api/jellyseerr.ts

@@ -0,0 +1,32 @@
+import { get } from "./shared";
+
+export interface JellyseerStat {
+	key: string;
+	label: string;
+	value: number;
+}
+
+export interface JellyseerRecentRequest {
+	id?: number | string;
+	type?: number | string;
+	name?: string;
+	status?: string;
+	media_status?: string;
+	created_at?: number | string;
+}
+
+export interface JellyseerrStatsResponse {
+	stats: JellyseerStat[];
+	recent: JellyseerRecentRequest[];
+	detail?: string | null;
+}
+
+/** Fetch Jellyseerr request stats for a Jellyfin service instance. */
+export async function fetchJellyseerrStats(
+	jellyfinServiceId?: string,
+): Promise<JellyseerrStatsResponse> {
+	return get<JellyseerrStatsResponse>(
+		"/api/jellyseerr/stats",
+		jellyfinServiceId ? { jellyfin_service_id: jellyfinServiceId } : undefined,
+	);
+}