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/FeedViewProvider.tsx

@@ -7,6 +7,7 @@ import {
   useState,
   type ReactNode,
 } from "react";
+import { qk } from "../lib/queryKeys";
 import { useQuery } from "@tanstack/react-query";
 import { api } from "../lib/api";
 import { parseFeedView, type FeedView } from "../lib/feedView";
@@ -19,7 +20,7 @@ import { LS, readAccount, writeAccount } from "../lib/storage";
 // The view mode is an account PREFERENCE, not a filter: it persists to the server on pick (savePrefs)
 // and is adopted from server prefs on login — so unlike the ephemeral list filters, this provider
 // carries the same server-sync the App god component used to. A localStorage mirror seeds the initial
-// render synchronously; the passive ["me"] observer adopts the server value once the account resolves.
+// render synchronously; the passive qk.me() observer adopts the server value once the account resolves.
 
 interface FeedViewActions {
   setView: (v: FeedView) => void;
@@ -40,7 +41,7 @@ export function FeedViewProvider({ children }: { children: ReactNode }) {
     parseFeedView(readAccount(LS.feedView, null)),
   );
   // Passive observer of the account App fetches (enabled:false → reads cache, re-renders on update).
-  const { data: me } = useQuery({ queryKey: ["me"], queryFn: api.me, enabled: false });
+  const { data: me } = useQuery({ queryKey: qk.me(), queryFn: api.me, enabled: false });
 
   // A user pick persists both to the localStorage mirror (instant reload) and to the server.
   const setView = useCallback((next: FeedView) => {