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/usePlaylistPreview.ts

@@ -0,0 +1,29 @@
+import { useCallback, useEffect } from 'react';
+import { usePreviewStore } from '../store/previewStore';
+import type { SubsonicSong } from '../api/subsonicTypes';
+
+export function usePlaylistPreview(): {
+  startPreview: (song: SubsonicSong) => void;
+} {
+  // Pause/resume of the main player + timer + cancel-on-supersede are all
+  // handled in `audio_preview_play` / `audio_preview_stop`. The store mirrors
+  // engine events so we just dispatch here and read `previewingId` for UI.
+  const startPreview = useCallback((song: SubsonicSong) => {
+    usePreviewStore.getState().startPreview({
+      id: song.id,
+      title: song.title,
+      artist: song.artist,
+      coverArt: song.coverArt,
+      duration: song.duration,
+    }, 'suggestions').catch(() => { /* engine errored — store already rolled back */ });
+  }, []);
+
+  // Cancel any in-flight preview when the user navigates away.
+  useEffect(() => () => {
+    if (usePreviewStore.getState().previewingId) {
+      usePreviewStore.getState().stopPreview();
+    }
+  }, []);
+
+  return { startPreview };
+}