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/build.gradle.kts

@@ -1,24 +0,0 @@
-allprojects {
-    repositories {
-        google()
-        mavenCentral()
-    }
-}
-
-val newBuildDir: Directory =
-    rootProject.layout.buildDirectory
-        .dir("../../build")
-        .get()
-rootProject.layout.buildDirectory.value(newBuildDir)
-
-subprojects {
-    val newSubprojectBuildDir: Directory = newBuildDir.dir(project.name)
-    project.layout.buildDirectory.value(newSubprojectBuildDir)
-}
-subprojects {
-    project.evaluationDependsOn(":app")
-}
-
-tasks.register<Delete>("clean") {
-    delete(rootProject.layout.buildDirectory)
-}