Add auto-scroll, file dialog, and transcript editing

- Auto-scroll transcript to active segment during playback with smart
  pause when user manually scrolls (resumes after 3s)
- Replace prompt() with native Tauri file dialog for audio/video import
  with file type filters
- Add inline transcript editing via double-click with Enter to save,
  Esc to cancel, preserving original text for change tracking
- Show "edited" badge on modified segments

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

src-tauri/src/lib.rs

@@ -10,6 +10,7 @@ use commands::transcribe::transcribe_file;
 pub fn run() {
     tauri::Builder::default()
         .plugin(tauri_plugin_opener::init())
+        .plugin(tauri_plugin_dialog::init())
         .invoke_handler(tauri::generate_handler![
             create_project,
             get_project,