Fix mobile app submodule initialization and cleanup

- Removed incorrect swingmusic_mobile directory
- Properly initialized swingmusic_mobile submodule from swingmusic-mobile.git
- Clean submodule configuration for unified release workflow

swingmusic_mobile/linux/runner/CMakeLists.txt

@@ -1,26 +0,0 @@
-cmake_minimum_required(VERSION 3.13)
-project(runner LANGUAGES CXX)
-
-# Define the application target. To change its name, change BINARY_NAME in the
-# top-level CMakeLists.txt, not the value here, or `flutter run` will no longer
-# work.
-#
-# Any new source files that you add to the application should be added here.
-add_executable(${BINARY_NAME}
-  "main.cc"
-  "my_application.cc"
-  "${FLUTTER_MANAGED_DIR}/generated_plugin_registrant.cc"
-)
-
-# Apply the standard set of build settings. This can be removed for applications
-# that need different build settings.
-apply_standard_settings(${BINARY_NAME})
-
-# Add preprocessor definitions for the application ID.
-add_definitions(-DAPPLICATION_ID="${APPLICATION_ID}")
-
-# Add dependency libraries. Add any application-specific dependencies here.
-target_link_libraries(${BINARY_NAME} PRIVATE flutter)
-target_link_libraries(${BINARY_NAME} PRIVATE PkgConfig::GTK)
-
-target_include_directories(${BINARY_NAME} PRIVATE "${CMAKE_SOURCE_DIR}")