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/lib/messaging.ts

@@ -1,5 +1,6 @@
 // Shared messaging helpers used by both the Messages page and the floating chat dock.
 import { useSyncExternalStore } from "react";
+import { qk } from "./queryKeys";
 import { useQuery } from "@tanstack/react-query";
 import { api } from "./api";
 import { forgetConv, isUnlocked, subscribeUnlock } from "./e2ee";
@@ -57,7 +58,7 @@ function useUnlocked(): boolean {
 export type KeyState = "loading" | "setup" | "unlock" | "ready";
 export function useKeyState(): KeyState {
   const unlocked = useUnlocked();
-  const q = useQuery({ queryKey: ["message-key"], queryFn: api.messageMyKey, staleTime: 60_000 });
+  const q = useQuery({ queryKey: qk.messageKey(), queryFn: api.messageMyKey, staleTime: 60_000 });
   if (q.isLoading && !q.data) return "loading";
   if (!(q.data?.configured ?? false)) return "setup";
   if (!unlocked) return "unlock";