Extract audio from video files before loading

Video files (MP4, MKV, etc.) are now processed with ffmpeg to extract
audio to a temp WAV file before loading into wavesurfer. This prevents
the WebView crash caused by trying to fetch multi-GB files into memory.

- New extract_audio Tauri command uses ffmpeg (sidecar-bundled or system)
- Frontend detects video extensions and extracts audio automatically
- User-friendly error if ffmpeg is not installed with install instructions
- Reverted wavesurfer MediaElement approach in favor of clean extraction
- Added FFmpeg install guide to USER_GUIDE.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/components/WaveformPlayer.svelte

@@ -57,6 +57,12 @@
       isReady = false;
     });
 
+    wavesurfer.on('error', (err: Error) => {
+      console.error('[voice-to-notes] WaveSurfer error:', err);
+      isLoading = false;
+      loadError = 'Failed to load audio';
+    });
+
     if (audioUrl) {
       loadAudio(audioUrl);
     }