chore(mobile): gitignore machine-specific generated files

Remove local.properties, Generated.xcconfig, and
flutter_export_environment.sh from tracking. These contain
absolute paths and are marked "do not check into version control".

Author: gahyun-git

.gitignore

@@ -58,6 +58,9 @@ apps/mobile/build/
 apps/mobile/.flutter-plugins
 apps/mobile/.flutter-plugins-dependencies
 apps/mobile/.packages
+apps/mobile/android/local.properties
+apps/mobile/ios/Flutter/Generated.xcconfig
+apps/mobile/ios/Flutter/flutter_export_environment.sh
 
 # Misc
 *.bak