feat(services): select Jellyfin via jellyfin_service_id on the frontend

Slice 4b frontend half. Jellyfin-touching pages now select a Jellyfin service
instance instead of a machine.

- api/client.ts: Jellyfin-backed calls (counts/libraries/activity/users, media
  status/build/stop/force-stop, queryMedia) send jellyfin_service_id.
- hooks/useDashboard, useUsers, useMedia: selector param renamed to
  jellyfinServiceId.
- pages/Media + Applications: list jellyfin service instances and persist
  jellyfin_service_id in the URL.
- Dashboard (widgets) and Users (default instance) need no selector change.
- Update Applications + Media tests for the new hook/param.

Files/SSH transport keeps machine_id. Verification: frontend lint 0 errors,
build success, 70 tests; backend ruff clean, 222 tests.

frontend/src/pages/__tests__/Applications.test.tsx

@@ -26,6 +26,12 @@ vi.mock("../../hooks/useSettings", () => ({
 	}),
 }));
 
+vi.mock("../../hooks/useServices", () => ({
+	useServiceInstances: () => ({
+		data: [{ id: "jfs1", service_type: "jellyfin", name: "Main", enabled: true }],
+	}),
+}));
+
 vi.mock("../../hooks/useDashboard", () => ({
 	useCounts: () => ({
 		data: { movies: 10, series: 5, episodes: 100 },