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/utils/runPlaylistLoad.ts

@@ -0,0 +1,38 @@
+import type React from 'react';
+import { getPlaylist } from '../api/subsonicPlaylists';
+import { filterSongsToActiveLibrary } from '../api/subsonicLibrary';
+import type { SubsonicPlaylist, SubsonicSong } from '../api/subsonicTypes';
+
+export interface RunPlaylistLoadDeps {
+  id: string;
+  setLoading: (v: boolean) => void;
+  setPlaylist: React.Dispatch<React.SetStateAction<SubsonicPlaylist | null>>;
+  setSongs: React.Dispatch<React.SetStateAction<SubsonicSong[]>>;
+  setCustomCoverId: React.Dispatch<React.SetStateAction<string | null>>;
+  setRatings: React.Dispatch<React.SetStateAction<Record<string, number>>>;
+  setStarredSongs: React.Dispatch<React.SetStateAction<Set<string>>>;
+}
+
+export async function runPlaylistLoad(deps: RunPlaylistLoadDeps): Promise<void> {
+  const { id, setLoading, setPlaylist, setSongs, setCustomCoverId, setRatings, setStarredSongs } = deps;
+  setLoading(true);
+  try {
+    const { playlist, songs } = await getPlaylist(id);
+    const filteredSongs = await filterSongsToActiveLibrary(songs);
+    setPlaylist(playlist);
+    setSongs(filteredSongs);
+    if (playlist.coverArt) setCustomCoverId(playlist.coverArt);
+    const init: Record<string, number> = {};
+    const starred = new Set<string>();
+    filteredSongs.forEach(s => {
+      if (s.userRating) init[s.id] = s.userRating;
+      if (s.starred) starred.add(s.id);
+    });
+    setRatings(init);
+    setStarredSongs(starred);
+  } catch {
+    // intentional swallow; load failure leaves loading false + playlist null
+  } finally {
+    setLoading(false);
+  }
+}