Playlist: per-track start/end trim, total/elapsed/remaining times, waveform cards

- Per-track start/end points (mm:ss inputs, 'set to current' buttons, draggable
  handles on the waveform). Trim applied via audio.setTrim (seek to start, stop
  at end) and pushed live to the playing track.
- Wider cards showing elapsed / total / remaining for the trimmed range.
- Waveform rendered per track: peaks computed in main via ffmpeg (mono 8kHz PCM
  -> 400 normalized buckets, cached), drawn on a canvas with trim region +
  playhead. New audio:peaks IPC + djv.peaks preload binding.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

preload.js

@@ -34,5 +34,6 @@ contextBridge.exposeInMainWorld('djv', {
   loadPlaylist: () => ipcRenderer.invoke('playlist:load'),
   savePlaylist: (data) => ipcRenderer.invoke('playlist:save', data),
   exportPlaylist: (data) => ipcRenderer.invoke('playlist:export', data),
-  importPlaylist: () => ipcRenderer.invoke('playlist:import')
+  importPlaylist: () => ipcRenderer.invoke('playlist:import'),
+  peaks: (path, buckets) => ipcRenderer.invoke('audio:peaks', path, buckets)
 });