Waveform: decode peaks in-renderer via Web Audio (no external ffmpeg needed)

The packaged app doesn't bundle ffmpeg, so on Macs without a system ffmpeg the
waveform stayed blank. Compute peaks by reading the file bytes (new file:read
IPC) and decoding with Web Audio's decodeAudioData in a low-rate AudioContext;
ffmpeg in main remains only as a fallback.

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

preload.js

@@ -35,5 +35,6 @@ contextBridge.exposeInMainWorld('djv', {
   savePlaylist: (data) => ipcRenderer.invoke('playlist:save', data),
   exportPlaylist: (data) => ipcRenderer.invoke('playlist:export', data),
   importPlaylist: () => ipcRenderer.invoke('playlist:import'),
-  peaks: (path, buckets) => ipcRenderer.invoke('audio:peaks', path, buckets)
+  peaks: (path, buckets) => ipcRenderer.invoke('audio:peaks', path, buckets),
+  readFile: (path) => ipcRenderer.invoke('file:read', path)
 });