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/About.tsx

@@ -1,4 +1,5 @@
 import { useQuery } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { Info, Sparkles } from "lucide-react";
 import { api } from "../lib/api";
@@ -21,7 +22,11 @@ export default function About({
   onOpenReleaseNotes: () => void;
 }) {
   const { t } = useTranslation();
-  const { data } = useQuery({ queryKey: ["version"], queryFn: api.version, staleTime: 5 * 60_000 });
+  const { data } = useQuery({
+    queryKey: qk.version(),
+    queryFn: api.version,
+    staleTime: 5 * 60_000,
+  });
 
   const rows: [string, string][] = [
     [