Add Kotlin Multiplatform mobile client

A Compose Multiplatform app (mobile/) targeting Android, iOS, and desktop,
sharing one Kotlin codebase: OIDC login via PKCE against the same Zitadel
instance the web player uses (loopback redirect on desktop, a musify://
custom scheme on mobile), a token store per platform, and a first pass at
browsing/streaming tracks through the existing API.

deploy/compose.yml's zitadel-init gains a third OIDC app registration
("Musify Native", public client, no secret) alongside the existing API and
web apps, writing its client id to NATIVE_CLIENT_ID in the env file the same
way the other two already do.

mobile/build.gradle.kts

@@ -0,0 +1,7 @@
+plugins {
+    alias(libs.plugins.kotlinMultiplatform) apply false
+    alias(libs.plugins.androidApplication) apply false
+    alias(libs.plugins.composeMultiplatform) apply false
+    alias(libs.plugins.composeCompiler) apply false
+    alias(libs.plugins.kotlinSerialization) apply false
+}