refactor(settings): drop the divider above the import dialog's action buttons

Remove the separator between the content and the Import/Sweep + Close buttons;
the spacing alone separates them cleanly enough.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app.cpp

@@ -3143,8 +3143,6 @@ void App::renderImportKeyDialog()
         ImGui::PopTextWrapPos();
     }
 
-    ImGui::Spacing();
-    ImGui::Separator();
     ImGui::Spacing();
 
     float btnW = 130.0f * dp;