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/hooks/useUsers.ts

@@ -2,10 +2,10 @@ import { useQuery } from "@tanstack/react-query";
 import { fetchUsers } from "../api/client";
 import type { UserDirectoryResponse } from "../types";
 
-export function useUsers(machineId?: string) {
+export function useUsers(jellyfinServiceId?: string) {
 	return useQuery<UserDirectoryResponse>({
-		queryKey: ["users", machineId ?? "default"],
-		queryFn: () => fetchUsers(machineId),
+		queryKey: ["users", jellyfinServiceId ?? "default"],
+		queryFn: () => fetchUsers(jellyfinServiceId),
 		staleTime: 30_000,
 	});
 }