feat(gui): Add 'Open in External Editor' button to patch modal

- Added button to launch external editor for reviewing agent proposed changes
- Added _open_patch_in_external_editor method to handle the launch logic
- Integrated with ExternalEditorLauncher and create_temp_modified_file

src/gui_2.py

@@ -2179,6 +2179,9 @@ class App:
       imgui.text(line)
    imgui.end_child()
    imgui.separator()
+   if imgui.button("Open in External Editor"):
+    self._open_patch_in_external_editor()
+   imgui.same_line()
    if imgui.button("Apply Patch"):
     self._apply_pending_patch()
    imgui.same_line()
@@ -2209,6 +2212,31 @@ class App:
   except Exception as e:
    self._patch_error_message = str(e)
 
+ def _open_patch_in_external_editor(self) -> None:
+  try:
+   from src.external_editor import get_default_launcher, create_temp_modified_file
+   import os
+   if not self._pending_patch_files:
+    self._patch_error_message = "No files to edit"
+    return
+   launcher = get_default_launcher()
+   editor = launcher.config.get_default()
+   if not editor:
+    self._patch_error_message = "No external editor configured"
+    return
+   original_path = self._pending_patch_files[0]
+   if not os.path.exists(original_path):
+    self._patch_error_message = f"Original file not found: {original_path}"
+    return
+   temp_path = create_temp_modified_file(self._pending_patch_text)
+   result = launcher.launch_diff(None, original_path, temp_path)
+   if result is None:
+    self._patch_error_message = "Failed to launch external editor"
+   else:
+    self._patch_error_message = None
+  except Exception as e:
+   self._patch_error_message = str(e)
+
  def request_patch_from_tier4(self, error: str, file_context: str) -> None:
   try:
    from src import ai_client