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/composeApp/src/desktopMain/kotlin/com/musify/app/Main.kt

@@ -0,0 +1,18 @@
+package com.musify.app
+
+import androidx.compose.ui.window.Window
+import androidx.compose.ui.window.application
+import androidx.compose.ui.unit.DpSize
+import androidx.compose.ui.window.WindowPosition
+import androidx.compose.ui.window.WindowState
+import androidx.compose.ui.unit.dp
+
+fun main() = application {
+    Window(
+        onCloseRequest = ::exitApplication,
+        title = "Musify",
+        state = WindowState(size = DpSize(420.dp, 760.dp), position = WindowPosition(alignment = androidx.compose.ui.Alignment.Center))
+    ) {
+        App()
+    }
+}