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/useMe.ts

@@ -1,13 +1,14 @@
 import { useQuery } from "@tanstack/react-query";
+import { qk } from "./queryKeys";
 import { api, type Me } from "./api";
 
-// The signed-in account, read from the ["me"] query cache. App owns the FETCHING query (gated on
+// The signed-in account, read from the qk.me() query cache. App owns the FETCHING query (gated on
 // setup state) and renders modules only once it has resolved to a real user — so anywhere inside the
 // authenticated module tree the cache is always populated. `enabled:false` makes this a passive
 // observer (no second fetch, which could 503 in setup mode) that still re-renders when the cache
 // updates (login, the 60s refetch, a Google-link round-trip). Lets modules read `me` from a hook
 // instead of having it prop-drilled through App.
 export function useMe(): Me {
-  const { data } = useQuery({ queryKey: ["me"], queryFn: api.me, enabled: false });
+  const { data } = useQuery({ queryKey: qk.me(), queryFn: api.me, enabled: false });
   return data as Me;
 }