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/android/.gitignore

@@ -1,14 +0,0 @@
-gradle-wrapper.jar
-/.gradle
-/captures/
-/gradlew
-/gradlew.bat
-/local.properties
-GeneratedPluginRegistrant.java
-.cxx/
-
-# Remember to never publicly share your keystore.
-# See https://flutter.dev/to/reference-keystore
-key.properties
-**/*.keystore
-**/*.jks