refactor(playlist): G.69 — extract runPlaylistLoad + usePlaylistPreview + usePlaylistBulkPlayCallbacks + usePlaylistDerived (cluster) (#636)

Four-cut cluster pulling the remaining handler bodies + memo island
out of PlaylistDetail.tsx. 507 → 437 LOC (−70).

runPlaylistLoad — async fetch + populate flow that the load
useEffect drives: getPlaylist → filterSongsToActiveLibrary →
setPlaylist + setSongs + setCustomCoverId from playlist.coverArt +
rebuild ratings + starredSongs from each song's stored userRating /
starred flag. Takes the six setters as deps. Page keeps the
useEffect that calls it on id / lastModified change.

usePlaylistPreview — startPreview callback (dispatches into
previewStore with the 'suggestions' source) plus the unmount-time
stopPreview cleanup. Returns just { startPreview }. No props
needed.

usePlaylistBulkPlayCallbacks — wraps playPlaylistAll /
shufflePlaylistAll / enqueuePlaylistAll utilities behind three
useCallback handles with the right deps array. Takes
{ songsLength, id, tracks, touchPlaylist, playTrack, enqueue }.

usePlaylistDerived — existingIds, tracks (songToTrack), sorted +
filtered displayedSongs via getDisplayedSongs, displayedTracks (with
the cheap aliasing optimization when displayedSongs === songs),
isFiltered. Subscribes to playerStore's userRatingOverrides +
starredOverrides directly so the page no longer threads them through
the memo deps.

PlaylistDetail drops these direct imports — they followed the new
modules: getPlaylist, filterSongsToActiveLibrary, songToTrack,
useMemo, getDisplayedSongs (now type-only),
playPlaylistAll/shuffle/enqueue from playlistBulkPlayActions.
Pure code move otherwise.

src/hooks/usePlaylistDerived.ts

@@ -0,0 +1,46 @@
+import { useMemo } from 'react';
+import type { SubsonicSong } from '../api/subsonicTypes';
+import type { Track } from '../store/playerStoreTypes';
+import { usePlayerStore } from '../store/playerStore';
+import { songToTrack } from '../utils/songToTrack';
+import { getDisplayedSongs, type PlaylistSortDir, type PlaylistSortKey } from '../utils/playlistDisplayedSongs';
+
+export interface PlaylistDerivedOptions {
+  filterText: string;
+  sortKey: PlaylistSortKey;
+  sortDir: PlaylistSortDir;
+  ratings: Record<string, number>;
+  starredSongs: Set<string>;
+}
+
+export interface PlaylistDerived {
+  existingIds: Set<string>;
+  tracks: Track[];
+  displayedSongs: SubsonicSong[];
+  displayedTracks: Track[];
+  isFiltered: boolean;
+}
+
+export function usePlaylistDerived(songs: SubsonicSong[], opts: PlaylistDerivedOptions): PlaylistDerived {
+  const userRatingOverrides = usePlayerStore(s => s.userRatingOverrides);
+  const starredOverrides = usePlayerStore(s => s.starredOverrides);
+  const { filterText, sortKey, sortDir, ratings, starredSongs } = opts;
+
+  const existingIds = useMemo(() => new Set(songs.map(s => s.id)), [songs]);
+  const tracks = useMemo(() => songs.map(songToTrack), [songs]);
+
+  const displayedSongs = useMemo(
+    () => getDisplayedSongs(songs, {
+      filterText, sortKey, sortDir,
+      ratings, userRatingOverrides, starredOverrides, starredSongs,
+    }),
+    [songs, filterText, sortKey, sortDir, ratings, userRatingOverrides, starredOverrides, starredSongs],
+  );
+  const displayedTracks = useMemo(
+    () => displayedSongs === songs ? tracks : displayedSongs.map(songToTrack),
+    [displayedSongs, songs, tracks],
+  );
+  const isFiltered = displayedSongs !== songs;
+
+  return { existingIds, tracks, displayedSongs, displayedTracks, isFiltered };
+}