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/capabilities/default.json

@@ -5,6 +5,7 @@
   "windows": ["main"],
   "permissions": [
     "core:default",
-    "opener:default"
+    "opener:default",
+    "dialog:default"
   ]
 }