refactor(fe): query-key factory + migrate feed/channels/playlists (R7 S2a)

Introduce lib/queryKeys.ts as the single source of truth for React Query
keys — the R7 driver was ~200 hand-written key literals across 51 files
(`["feed"]` alone in 15+), where one typo silently orphans a key so an
invalidate never matches.

S2a migrates the three highest-traffic domains (feed/video, channels,
playlists) plus the shared me/my-status/tags roots: ~90 literal sites in
18 files now call qk.*(). Pure substitution — each factory fn returns the
byte-identical array the call sites used before, so TanStack's prefix
matching is unchanged (qk.feed() still invalidates qk.feed(filters)).
Co-invalidation clusters are deliberately NOT bundled into helpers (they
differ per site), so no query's refetch scope changes.

Nullable keys (playlist(selectedId), ytSearch(q)) accept null so a null
segment is preserved (["playlist", null]) while a no-arg call is the bare
prefix key — guard is `!== undefined`.

Gate green: typecheck, eslint (0 err), prettier, 68 vitest. Smoke: app
boots, feed/facets/tags/my-status all 200, no console errors. Factory
extended to plex/messaging/admin/downloads/notifications in S2b.

frontend/src/components/ChannelPicker.tsx

@@ -1,6 +1,7 @@
 import { useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useQuery } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
 import { X } from "lucide-react";
 import { api, type FeedFilters } from "../lib/api";
 import { useScrollFade } from "../lib/useScrollFade";
@@ -25,7 +26,7 @@ export default function ChannelPicker({
   // Same edge-fade affordance the nav rail and the panel body use to hint at more content.
   const fade = useScrollFade();
   const channelsQuery = useQuery({
-    queryKey: ["channels"],
+    queryKey: qk.channels(),
     queryFn: api.channels,
     staleTime: 5 * 60_000,
   });