feat(prefs): usePref — one per-account+server preference hook (E4 S1)

Fold the hand-rolled server-pref plumbing (savePrefs + local mirror + me-cache
merge + adopt-on-login) into a single usePref<T>(serverKey, default, opts) hook,
plus useSavePref for dynamic multi-key patches.

Migrated (6 savePrefs call sites + 2 duplicated adopt effects -> the hook):
- FeedViewProvider: view
- App: filterCollapsed, playlistsCollapsed (+ their adopt block removed)
- PlayerModal: playerAutoAdvance, playerLoop
- plexDetailUi: plexInfoArtBg/Cast/Related (savePref via useSavePref)

Left as-is by design: panel layouts (a dynamically-indexed Record, not one key),
language (i18n store, no React state), PrefsProvider (debounced multi-field
auto-save + baseline echo-suppression - a different model).

Behaviour-equivalent (net -84 LOC at the call sites). usePref.dom.test covers
the three seed sources, persist path, debounce, adopt, parse, and useSavePref.
Frontend gate green: typecheck, knip, eslint (0 err), prettier, 238 vitest.

frontend/src/lib/plexDetailUi.tsx

@@ -1,10 +1,8 @@
 import { useLayoutEffect, useRef, useState, type ReactNode } from "react";
-import { qk } from "./queryKeys";
 import { useTranslation } from "react-i18next";
-import { useQueryClient } from "@tanstack/react-query";
 import { SlidersHorizontal } from "lucide-react";
 import { useDismiss } from "./useDismiss";
-import { api } from "./api";
+import { usePref, useSavePref } from "./usePref";
 
 // Shared "detail page" UI reused by the movie info page (PlexInfo) and the series show/season pages,
 // so they look consistent (the standing glassy/HTPC brief): the faint fixed art backdrop, the two
@@ -12,42 +10,21 @@ import { api } from "./api";
 // customize menu that toggles them.
 
 export function useDetailPrefs() {
-  const qc = useQueryClient();
-  const prefs = (qc.getQueryData<{ preferences?: Record<string, unknown> }>(qk.me())?.preferences ??
-    {}) as {
-    plexInfoArtBg?: boolean;
-    plexInfoCast?: boolean;
-    plexInfoRelated?: boolean;
-  };
-  const [artBg, setArtBg] = useState(prefs.plexInfoArtBg !== false);
-  const [showCast, setShowCast] = useState(prefs.plexInfoCast !== false);
-  const [showRelated, setShowRelated] = useState(prefs.plexInfoRelated !== false);
-  const save = (patch: Record<string, unknown>) => {
-    api.savePrefs(patch).catch(() => {});
-    qc.setQueryData<{ preferences?: Record<string, unknown> } | undefined>(qk.me(), (m) =>
-      m ? { ...m, preferences: { ...(m.preferences || {}), ...patch } } : m,
-    );
-  };
+  // Three per-user detail-page toggles, each a server pref adopted on login (usePref). Default ON:
+  // undefined → true, only an explicit false hides. The detail pages mount after `me` is cached, so
+  // usePref seeds from the cache (no localStorage mirror).
+  const [artBg, setArtBg] = usePref<boolean>("plexInfoArtBg", true);
+  const [showCast, setShowCast] = usePref<boolean>("plexInfoCast", true);
+  const [showRelated, setShowRelated] = usePref<boolean>("plexInfoRelated", true);
+  const savePref = useSavePref();
   return {
     artBg,
     showCast,
     showRelated,
-    savePref: save, // expose the raw persister for callers with extra per-user prefs (e.g. strip sources)
-    toggleArtBg: () => {
-      const next = !artBg;
-      setArtBg(next);
-      save({ plexInfoArtBg: next });
-    },
-    toggleCast: () => {
-      const next = !showCast;
-      setShowCast(next);
-      save({ plexInfoCast: next });
-    },
-    toggleRelated: () => {
-      const next = !showRelated;
-      setShowRelated(next);
-      save({ plexInfoRelated: next });
-    },
+    savePref, // the raw persister for callers with extra per-user prefs (e.g. strip sources)
+    toggleArtBg: () => setArtBg(!artBg),
+    toggleCast: () => setShowCast(!showCast),
+    toggleRelated: () => setShowRelated(!showRelated),
   };
 }