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>

docs/USER_GUIDE.md

@@ -26,10 +26,13 @@ The sidecar only needs to be downloaded once. Updates are detected automatically
 
 ## Basic Workflow
 
-### 1. Import Audio
+### 1. Import Audio or Video
 
 - Click **Import Audio** or press **Ctrl+O** (Cmd+O on Mac)
-- Supported formats: MP3, WAV, FLAC, OGG, M4A, AAC, WMA, MP4, MKV, AVI, MOV, WebM
+- **Audio formats:** MP3, WAV, FLAC, OGG, M4A, AAC, WMA
+- **Video formats:** MP4, MKV, AVI, MOV, WebM — audio is automatically extracted
+
+> **Note:** Video file import requires [FFmpeg](#installing-ffmpeg) to be installed on your system.
 
 ### 2. Transcribe
 
@@ -181,8 +184,42 @@ If you prefer cloud-based AI:
 
 ---
 
+## Installing FFmpeg
+
+FFmpeg is required for importing video files (MP4, MKV, AVI, etc.). It's used to extract the audio track before transcription.
+
+**Windows:**
+```
+winget install ffmpeg
+```
+Or download from [ffmpeg.org/download.html](https://ffmpeg.org/download.html) and add to your PATH.
+
+**macOS:**
+```
+brew install ffmpeg
+```
+
+**Linux (Debian/Ubuntu):**
+```
+sudo apt install ffmpeg
+```
+
+**Linux (Fedora/RHEL):**
+```
+sudo dnf install ffmpeg
+```
+
+After installing, restart Voice to Notes. FFmpeg is not needed for audio-only files (MP3, WAV, FLAC, etc.).
+
+---
+
 ## Troubleshooting
 
+### Video import fails / "FFmpeg not found"
+- Install FFmpeg using the instructions above
+- Make sure `ffmpeg` is in your system PATH
+- Restart Voice to Notes after installing
+
 ### Transcription is slow
 - Use a smaller model (tiny or base)
 - If you have an NVIDIA GPU, select CUDA in Settings > Transcription > Device