refactor(fe): migrate plex/messaging/admin/downloads/notifications to qk (R7 S2b)

Extend lib/queryKeys.ts to the remaining ~34 key roots and migrate every
remaining literal (queryKey: [...] AND the positional useLiveQuery keys) in
36 files. All ~200 React Query keys across the app now route through the
factory; zero raw key literals remain outside queryKeys.ts.

The heterogeneous composite plex keys (plex-library/facets/collections/
playlists carry a filters object, a ratingKeys array, or a union/group
discriminator) take a variadic pass-through — the factory centralizes the
ROOT string; single-id keys stay precisely typed. Nullable keys (thread,
playlist, ytSearch) accept null so a null segment is preserved.

Pure substitution — byte-identical key arrays, so TanStack prefix matching
is unchanged. Gate green: typecheck (app+node+e2e), eslint (0 err),
prettier, 68 vitest. Runtime-verified on a fresh dev server: Feed + Plex
modules load, feed/facets/tags/my-status/plex-library/plex-facets/
plex-collections/plex-playlists all 200, full UI renders.

frontend/src/components/DownloadDialog.tsx

@@ -1,4 +1,5 @@
 import { lazy, Suspense, useState } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import Modal from "./Modal";
@@ -24,7 +25,7 @@ export default function DownloadDialog({
 }) {
   const { t } = useTranslation();
   const qc = useQueryClient();
-  const profilesQ = useQuery({ queryKey: ["download-profiles"], queryFn: api.downloadProfiles });
+  const profilesQ = useQuery({ queryKey: qk.downloadProfiles(), queryFn: api.downloadProfiles });
   const [profileId, setProfileId] = useState<number | null>(null);
   const [name, setName] = useState("");
   const [busy, setBusy] = useState(false);