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-lock.json

@@ -10,6 +10,7 @@
       "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",
@@ -1219,6 +1220,15 @@
         "node": ">= 10"
       }
     },
+    "node_modules/@tauri-apps/plugin-dialog": {
+      "version": "2.6.0",
+      "resolved": "https://registry.npmjs.org/@tauri-apps/plugin-dialog/-/plugin-dialog-2.6.0.tgz",
+      "integrity": "sha512-q4Uq3eY87TdcYzXACiYSPhmpBA76shgmQswGkSVio4C82Sz2W4iehe9TnKYwbq7weHiL88Yw19XZm7v28+Micg==",
+      "license": "MIT OR Apache-2.0",
+      "dependencies": {
+        "@tauri-apps/api": "^2.8.0"
+      }
+    },
     "node_modules/@tauri-apps/plugin-opener": {
       "version": "2.5.3",
       "resolved": "https://registry.npmjs.org/@tauri-apps/plugin-opener/-/plugin-opener-2.5.3.tgz",