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>

package.json

@@ -16,6 +16,7 @@
   "license": "MIT",
   "dependencies": {
     "@tauri-apps/api": "^2",
+    "@tauri-apps/plugin-dialog": "^2.6.0",
     "@tauri-apps/plugin-opener": "^2",
     "@tiptap/core": "^3.20.0",
     "@tiptap/pm": "^3.20.0",