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/Cargo.toml

@@ -22,3 +22,4 @@ rusqlite = { version = "0.31", features = ["bundled"] }
 uuid = { version = "1", features = ["v4", "serde"] }
 thiserror = "1"
 chrono = { version = "0.4", features = ["serde"] }
+tauri-plugin-dialog = "2.6.0"