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/SavedViewsWidget.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
 import { BookmarkPlus, Check, GripVertical, Pencil, Share2, Star, Trash2, X } from "lucide-react";
 import {
   closestCenter,
@@ -56,7 +57,7 @@ export default function SavedViewsWidget({
   const { t } = useTranslation();
   const qc = useQueryClient();
   const confirm = useConfirm();
-  const viewsQuery = useQuery({ queryKey: ["saved-views"], queryFn: api.savedViews });
+  const viewsQuery = useQuery({ queryKey: qk.savedViews(), queryFn: api.savedViews });
   const views = viewsQuery.data ?? [];
   const [naming, setNaming] = useState(false);
   const [draftName, setDraftName] = useState("");
@@ -69,7 +70,7 @@ export default function SavedViewsWidget({
     if (viewsQuery.data) syncDefaultMirror(viewsQuery.data);
   }, [viewsQuery.data]);
 
-  const refresh = () => qc.invalidateQueries({ queryKey: ["saved-views"] });
+  const refresh = () => qc.invalidateQueries({ queryKey: qk.savedViews() });
   const currentKey = keyOf(filters);
 
   async function save() {
@@ -124,7 +125,7 @@ export default function SavedViewsWidget({
     const next = arrayMove(ids, oldIndex, newIndex);
     // Optimistic reorder so the drop sticks visually before the round-trip.
     qc.setQueryData<SavedView[]>(
-      ["saved-views"],
+      qk.savedViews(),
       next.map((id) => views.find((v) => v.id === id)!),
     );
     await api.reorderViews(next);