cbb703341e
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.