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/NavSidebar.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useRef, useState, useSyncExternalStore } from "react";
+import { qk } from "../lib/queryKeys";
 import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
 import { useQuery } from "@tanstack/react-query";
@@ -114,7 +115,7 @@ export default function NavSidebar({
 
   // Accounts that have signed in on this browser (loaded only while the popover is open).
   const accountsQuery = useQuery({
-    queryKey: ["accounts"],
+    queryKey: qk.accounts(),
     queryFn: api.accounts,
     enabled: acctOpen,
   });
@@ -138,7 +139,7 @@ export default function NavSidebar({
 
   // Durable, server-backed unread count for the inbox badge. Polled live (pauses when the
   // tab is hidden); coexists with the client-side transient bell at the bottom of the rail.
-  const unreadQuery = useLiveQuery(["notif-unread"], api.notificationUnreadCount, {
+  const unreadQuery = useLiveQuery(qk.notifUnread(), api.notificationUnreadCount, {
     intervalMs: 30000,
   });
   // One indicator for both layers: durable server notifications + the client-side transient
@@ -146,14 +147,14 @@ export default function NavSidebar({
   const clientUnread = useSyncExternalStore(subscribe, getUnreadCount, getUnreadCount);
   const unread = (unreadQuery.data?.count ?? 0) + clientUnread;
   // Direct messages are their own module with their own unread badge (demo can't message).
-  const msgUnreadQuery = useLiveQuery(["message-unread"], api.messageUnreadCount, {
+  const msgUnreadQuery = useLiveQuery(qk.messageUnread(), api.messageUnreadCount, {
     intervalMs: 30000,
     enabled: !me.is_demo,
   });
   const msgUnread = msgUnreadQuery.data?.count ?? 0;
   // Download center badge = items still working (queued/running/paused). Reuses the same
   // lightweight per-user index the feed cards poll, so no extra request shape.
-  const dlIndexQuery = useLiveQuery(["download-index"], api.downloadIndex, {
+  const dlIndexQuery = useLiveQuery(qk.downloadIndex(), api.downloadIndex, {
     intervalMs: 30000,
     enabled: !me.is_demo,
   });