refactor(playlist): G.68 — extract runPlaylistSaveMeta + 3 hooks (song search / mutations / star+rating) (#635)

Four-cut cluster. 572 → 471 LOC (−101). Each handler on the page
collapses to a one-line delegate; pure code move otherwise.

runPlaylistSaveMeta — the meta save flow (updatePlaylistMeta then
optional uploadPlaylistCoverArt + getPlaylist refresh + cover toast,
or coverRemoved → null, then metaSaved toast + closes the modal).
Takes the deps separately from the opts object so the call-site
just passes through the modal's payload.

usePlaylistSongSearch — searchOpen + searchQuery driven debounced
search against subsonic. Owns the 350 ms timeout, the filter-out
of songs already in the playlist, and the searching state. Returns
{ searchResults, setSearchResults, searching } so addSong on the
page can still drop a just-added song out of the result list.

usePlaylistSongMutations — addSong / removeSong. removeSong is
trivial (filter + setSongs + savePlaylist with prevCount).
addSong preserves the .main-content scrollTop save / requestAnimationFrame
restore trick + drops the song out of both suggestions and search
results + fires the add toast with playlist name interpolation.

usePlaylistStarRating — handleRate (local override + playerStore
userRatingOverride + setRating API) + handleToggleStar (e.stopPropagation,
local set + playerStore starredOverride + star/unstar API). Reads
starredOverrides / setStarredOverride from playerStore directly so
the page doesn't have to thread them through.

PlaylistDetail drops these direct imports (now consumed in the
hooks/utility): updatePlaylistMeta, uploadPlaylistCoverArt, search,
setRating, star, unstar, showToast, useRef. Pure code move.

src/hooks/usePlaylistSongSearch.ts

@@ -0,0 +1,37 @@
+import { useEffect, useRef, useState } from 'react';
+import type React from 'react';
+import { search } from '../api/subsonicSearch';
+import type { SubsonicSong } from '../api/subsonicTypes';
+
+export interface PlaylistSongSearchResult {
+  searchResults: SubsonicSong[];
+  setSearchResults: React.Dispatch<React.SetStateAction<SubsonicSong[]>>;
+  searching: boolean;
+}
+
+export function usePlaylistSongSearch(
+  songs: SubsonicSong[],
+  searchOpen: boolean,
+  searchQuery: string,
+): PlaylistSongSearchResult {
+  const [searchResults, setSearchResults] = useState<SubsonicSong[]>([]);
+  const [searching, setSearching] = useState(false);
+  const searchDebounce = useRef<ReturnType<typeof setTimeout> | null>(null);
+
+  useEffect(() => {
+    if (!searchOpen || !searchQuery.trim()) { setSearchResults([]); return; }
+    if (searchDebounce.current) clearTimeout(searchDebounce.current);
+    searchDebounce.current = setTimeout(async () => {
+      setSearching(true);
+      try {
+        const res = await search(searchQuery, { songCount: 20, artistCount: 0, albumCount: 0 });
+        const existingIds = new Set(songs.map(s => s.id));
+        setSearchResults(res.songs.filter(s => !existingIds.has(s.id)));
+      } catch {}
+      setSearching(false);
+    }, 350);
+    return () => { if (searchDebounce.current) clearTimeout(searchDebounce.current); };
+  }, [searchQuery, searchOpen, songs]);
+
+  return { searchResults, setSearchResults, searching };
+}