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);
     }

src/routes/+page.svelte

@@ -254,6 +254,8 @@
     // Changes persist when user saves the project file.
   }
 
+  const VIDEO_EXTENSIONS = ['mp4', 'mkv', 'avi', 'mov', 'webm'];
+
   async function handleFileImport() {
     const filePath = await open({
       multiple: false,
@@ -265,9 +267,34 @@
     });
     if (!filePath) return;
 
-    // Track the original file path and convert to asset URL for wavesurfer
+    // For video files, extract audio first using ffmpeg
+    const ext = filePath.split('.').pop()?.toLowerCase() ?? '';
+    let audioPath = filePath;
+    if (VIDEO_EXTENSIONS.includes(ext)) {
+      try {
+        audioPath = await invoke<string>('extract_audio', { filePath });
+      } catch (err) {
+        console.error('[voice-to-notes] Failed to extract audio:', err);
+        const msg = String(err);
+        if (msg.includes('ffmpeg not found')) {
+          alert(
+            'FFmpeg is required to import video files.\n\n' +
+            'Install FFmpeg:\n' +
+            '  Windows: winget install ffmpeg\n' +
+            '  macOS: brew install ffmpeg\n' +
+            '  Linux: sudo apt install ffmpeg\n\n' +
+            'Then restart Voice to Notes and try again.'
+          );
+        } else {
+          alert(`Failed to extract audio from video: ${msg}`);
+        }
+        return;
+      }
+    }
+
+    // Track the original file path (video or audio) for the sidecar
     audioFilePath = filePath;
-    audioUrl = convertFileSrc(filePath);
+    audioUrl = convertFileSrc(audioPath);
     waveformPlayer?.loadAudio(audioUrl);
 
     // Clear previous results