Playlist-video blend/opacity/fit controls + export/import playlist to file

- Blend mode, opacity and fit for videos played from the playlist (mix with visuals).
- Save/Load buttons to export/import a playlist (with scenes/cues) to a JSON file
  via native dialogs; the playlist already auto-persists between sessions.

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

preload.js

@@ -32,5 +32,7 @@ contextBridge.exposeInMainWorld('djv', {
   loadPads: () => ipcRenderer.invoke('pads:load'),
   savePads: (data) => ipcRenderer.invoke('pads:save', data),
   loadPlaylist: () => ipcRenderer.invoke('playlist:load'),
-  savePlaylist: (data) => ipcRenderer.invoke('playlist:save', data)
+  savePlaylist: (data) => ipcRenderer.invoke('playlist:save', data),
+  exportPlaylist: (data) => ipcRenderer.invoke('playlist:export', data),
+  importPlaylist: () => ipcRenderer.invoke('playlist:import')
 });