refactor(utils): group utils/ files into topic folders (Phase L, part 1) (#689)

111 of 122 top-level src/utils/ files move into 16 topic folders (audio,
cache, cover, share, server, playback, playlist, deviceSync, waveform,
mix, format, export, changelog, ui, perf, componentHelpers). True
singletons with no cluster stay at the utils/ root.

Pure file-move: a path-aware codemod rewrote 539 relative-import
specifiers across 275 files; no logic touched. The hot-path coverage
gate list (.github/frontend-hot-path-files.txt) is updated to the new
paths for the 11 gated utils files — a mechanical consequence of the
move, not a CI change. tsc is green.

src/utils/playback/playAlbum.ts

@@ -0,0 +1,55 @@
+import { getAlbum } from '../../api/subsonicLibrary';
+import { usePlayerStore } from '../../store/playerStore';
+import { songToTrack } from './songToTrack';
+import { useOrbitStore } from '../../store/orbitStore';
+
+function fadeOut(setVolume: (v: number) => void, from: number, durationMs: number): Promise<void> {
+  return new Promise(resolve => {
+    const steps = 16;
+    const stepMs = durationMs / steps;
+    let step = 0;
+    const id = setInterval(() => {
+      step++;
+      setVolume(Math.max(0, from * (1 - step / steps)));
+      if (step >= steps) {
+        clearInterval(id);
+        resolve();
+      }
+    }, stepMs);
+  });
+}
+
+export async function playAlbum(albumId: string): Promise<void> {
+  const albumData = await getAlbum(albumId);
+  const albumGenre = albumData.album.genre;
+  const tracks = albumData.songs.map(s => {
+    const track = songToTrack(s);
+    if (!track.genre && albumGenre) track.genre = albumGenre;
+    return track;
+  });
+  if (!tracks.length) return;
+
+  // In Orbit sessions, playAlbum is effectively an append operation (the
+  // playerStore bulk-gate also routes replaces into enqueue). Skip the
+  // fadeOut entirely — the current track keeps playing, the album goes
+  // onto the end of the queue after the user confirms the bulk dialog.
+  const orbitRole = useOrbitStore.getState().role;
+  if (orbitRole === 'host' || orbitRole === 'guest') {
+    usePlayerStore.getState().enqueue(tracks);
+    return;
+  }
+
+  const store = usePlayerStore.getState();
+  const { isPlaying, volume } = store;
+
+  if (isPlaying) {
+    await fadeOut(store.setVolume, volume, 700);
+    // Restore volume only in the Zustand store — do NOT call audio_set_volume here,
+    // otherwise the old track glitches back to full volume before playTrack stops it.
+    // playTrack reads state.volume and passes it to audio_play, so the new track
+    // starts at the correct volume without the Rust engine ever hearing a restore.
+    usePlayerStore.setState({ volume });
+  }
+
+  usePlayerStore.getState().playTrack(tracks[0], tracks);
+}