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-tauri/src/commands/media.rs

@@ -0,0 +1,95 @@
+use std::path::PathBuf;
+use std::process::Command;
+
+/// Extract audio from a video file to a WAV file using ffmpeg.
+/// Returns the path to the extracted audio file.
+#[tauri::command]
+pub fn extract_audio(file_path: String) -> Result<String, String> {
+    let input = PathBuf::from(&file_path);
+    if !input.exists() {
+        return Err(format!("File not found: {}", file_path));
+    }
+
+    // Output to a temp WAV file next to the original or in temp dir
+    let stem = input.file_stem().unwrap_or_default().to_string_lossy();
+    let output = std::env::temp_dir().join(format!("{stem}_audio.wav"));
+
+    eprintln!(
+        "[media] Extracting audio: {} -> {}",
+        input.display(),
+        output.display()
+    );
+
+    // Find ffmpeg — check sidecar extract dir first, then system PATH
+    let ffmpeg = find_ffmpeg().ok_or("ffmpeg not found. Install ffmpeg or ensure it's in PATH.")?;
+
+    let status = Command::new(&ffmpeg)
+        .args([
+            "-y",             // Overwrite output
+            "-i",
+            &file_path,
+            "-vn",            // No video
+            "-acodec",
+            "pcm_s16le",      // WAV PCM 16-bit
+            "-ar",
+            "16000",          // 16kHz (optimal for whisper)
+            "-ac",
+            "1",              // Mono
+        ])
+        .arg(output.to_str().unwrap())
+        .stdout(std::process::Stdio::null())
+        .stderr(std::process::Stdio::piped())
+        .status()
+        .map_err(|e| format!("Failed to run ffmpeg: {e}"))?;
+
+    if !status.success() {
+        return Err(format!("ffmpeg exited with status {status}"));
+    }
+
+    if !output.exists() {
+        return Err("ffmpeg completed but output file not found".to_string());
+    }
+
+    eprintln!("[media] Audio extracted successfully");
+    Ok(output.to_string_lossy().to_string())
+}
+
+/// Find ffmpeg binary — check sidecar directory first, then system PATH.
+fn find_ffmpeg() -> Option<String> {
+    // Check sidecar extract dir (ffmpeg is bundled with the sidecar)
+    if let Some(data_dir) = crate::sidecar::DATA_DIR.get() {
+        // Read sidecar version to find the right directory
+        let version_file = data_dir.join("sidecar-version.txt");
+        if let Ok(version) = std::fs::read_to_string(&version_file) {
+            let version = version.trim();
+            let sidecar_dir = data_dir.join(format!("sidecar-{version}"));
+            let ffmpeg_name = if cfg!(target_os = "windows") {
+                "ffmpeg.exe"
+            } else {
+                "ffmpeg"
+            };
+            let ffmpeg_path = sidecar_dir.join(ffmpeg_name);
+            if ffmpeg_path.exists() {
+                return Some(ffmpeg_path.to_string_lossy().to_string());
+            }
+        }
+    }
+
+    // Fall back to system PATH
+    let ffmpeg_name = if cfg!(target_os = "windows") {
+        "ffmpeg.exe"
+    } else {
+        "ffmpeg"
+    };
+    if Command::new(ffmpeg_name)
+        .arg("-version")
+        .stdout(std::process::Stdio::null())
+        .stderr(std::process::Stdio::null())
+        .status()
+        .is_ok()
+    {
+        return Some(ffmpeg_name.to_string());
+    }
+
+    None
+}