Project folders, always-extract audio, re-link support

Projects now save as folders containing .vtn + audio.wav:
  My Transcript/
    My Transcript.vtn
    audio.wav

Audio handling:
- Always extract to 22kHz mono WAV on import (all formats, not just video)
- Prevents WebAudio crash from decoding large MP3/FLAC/OGG to PCM in memory
- WAV saved alongside .vtn on project save (moved from temp)
- Sidecar still uses original file (does its own conversion)

Project format v2:
- source_file: original import path (for re-extraction)
- audio_wav: relative path to extracted WAV (portable)

Re-link on open:
- If audio.wav exists → load directly
- If missing but source exists → re-extract automatically
- If both missing → dialog to locate file via file picker
- V1 project migration: extracts WAV on first open

New Rust commands: check_file_exists, copy_file, create_dir
extract_audio: now accepts optional output_path, uses 22kHz sample rate

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

src-tauri/src/commands/media.rs

@@ -7,15 +7,18 @@ use std::os::windows::process::CommandExt;
 /// 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> {
+pub fn extract_audio(file_path: String, output_path: Option<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
+    // Use provided output path, or fall back to a temp WAV file
     let stem = input.file_stem().unwrap_or_default().to_string_lossy();
-    let output = std::env::temp_dir().join(format!("{stem}_audio.wav"));
+    let output = match output_path {
+        Some(ref p) => PathBuf::from(p),
+        None => std::env::temp_dir().join(format!("{stem}_audio.wav")),
+    };
 
     eprintln!(
         "[media] Extracting audio: {} -> {}",
@@ -35,7 +38,7 @@ pub fn extract_audio(file_path: String) -> Result<String, String> {
             "-acodec",
             "pcm_s16le",      // WAV PCM 16-bit
             "-ar",
-            "16000",          // 16kHz (optimal for whisper)
+            "22050",          // 22kHz mono for better playback quality
             "-ac",
             "1",              // Mono
         ])
@@ -63,6 +66,23 @@ pub fn extract_audio(file_path: String) -> Result<String, String> {
     Ok(output.to_string_lossy().to_string())
 }
 
+#[tauri::command]
+pub fn check_file_exists(path: String) -> bool {
+    std::path::Path::new(&path).exists()
+}
+
+#[tauri::command]
+pub fn copy_file(src: String, dst: String) -> Result<(), String> {
+    std::fs::copy(&src, &dst).map_err(|e| format!("Failed to copy file: {e}"))?;
+    Ok(())
+}
+
+#[tauri::command]
+pub fn create_dir(path: String) -> Result<(), String> {
+    std::fs::create_dir_all(&path).map_err(|e| format!("Failed to create directory: {e}"))?;
+    Ok(())
+}
+
 /// 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)