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/App.tsx

@@ -1,4 +1,5 @@
 import { lazy, Suspense, useEffect, useState } from "react";
+import { qk } from "./lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { api, getActiveAccount, setActiveAccount, setUnauthorizedHandler } from "./lib/api";
@@ -135,7 +136,7 @@ export default function App() {
   // We check this before anything else and render the wizard; `me` is held back until we know the
   // instance is set up (otherwise it would 503 against the setup-mode lock).
   const setupQuery = useQuery({
-    queryKey: ["setup-status"],
+    queryKey: qk.setupStatus(),
     queryFn: api.setupStatus,
     staleTime: Infinity,
   });
@@ -145,7 +146,7 @@ export default function App() {
   // Only poll while signed in (data present) — polling on the public login page is pointless and
   // its periodic refetch caused a visible flicker there.
   const meQuery = useQuery({
-    queryKey: ["me"],
+    queryKey: qk.me(),
     queryFn: api.me,
     enabled: setupQuery.isSuccess && !needsSetup,
     refetchInterval: (query) => (query.state.data ? 60_000 : false),
@@ -173,7 +174,7 @@ export default function App() {
   // can't loop the public landing.
   useEffect(() => {
     setUnauthorizedHandler(() => {
-      if (queryClient.getQueryData(["me"])) {
+      if (queryClient.getQueryData(qk.me())) {
         queryClient.clear();
         window.location.reload();
       }
@@ -223,7 +224,7 @@ export default function App() {
 
   // On login, adopt the server-stored SHELL preferences (panel layouts, rail collapse flags,
   // language). The Settings-page prefs (theme/perf/hints/notifications) are adopted by PrefsProvider
-  // and the feed view by FeedViewProvider — each via its own passive ["me"] observer.
+  // and the feed view by FeedViewProvider — each via its own passive qk.me() observer.
   useEffect(() => {
     const prefs = meQuery.data?.preferences;
     if (!prefs) return;