fix: correct dropdown indentation and syntax in unit selector

Co-authored-by: aider (openrouter/deepseek/deepseek-r1:free) <aider@aider.chat>

lib/ai/unit_dropdown.dart

@@ -51,6 +51,7 @@ class UnitDropdown extends StatelessWidget {
       dropdownPadding: const EdgeInsets.symmetric(vertical: 8),
       menuMaxHeight: 400,
       isExpanded: true,
+      ),
     );
   }
 }