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.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

mobile/.gitignore

@@ -0,0 +1,14 @@
+# Gradle / Kotlin
+.gradle/
+build/
+.kotlin/
+local.properties
+
+# IDE
+.idea/
+*.iml
+
+# Xcode (if a companion iosApp project is added)
+*.xcodeproj/xcuserdata/
+*.xcworkspace/xcuserdata/
+DerivedData/