Timed scene cues per track + persistent playlist/scenes

- Each playlist track holds a timeline of cues (@time → effect/text/image);
  cues fire automatically as the track plays, driven by progress reports.
- Playlist (tracks, hotkeys, cues) is saved to userData and restored on startup,
  like the pad bank.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

preload.js

@@ -30,5 +30,7 @@ contextBridge.exposeInMainWorld('djv', {
   openRecordingsFolder: () => ipcRenderer.invoke('rec:openFolder'),
   // Pad bank persistence.
   loadPads: () => ipcRenderer.invoke('pads:load'),
-  savePads: (data) => ipcRenderer.invoke('pads:save', data)
+  savePads: (data) => ipcRenderer.invoke('pads:save', data),
+  loadPlaylist: () => ipcRenderer.invoke('playlist:load'),
+  savePlaylist: (data) => ipcRenderer.invoke('playlist:save', data)
 });