Add push notification support for Android and iOS

- Integrated Firebase Cloud Messaging (FCM) for Android
- Integrated Apple Push Notification Service (APNs) for iOS
- Created shared notification models and API client
- Added device registration and token management
- Added notification permission handling for Android
- Created PushNotificationManager for iOS with AppDelegate
- Added placeholder google-services.json (needs to be replaced with actual config)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

composeApp/build.gradle.kts

@@ -1,6 +1,7 @@
 import org.jetbrains.compose.desktop.application.dsl.TargetFormat
 import org.jetbrains.kotlin.gradle.ExperimentalWasmDsl
 import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.plugin.KotlinDependencyHandler
 
 plugins {
     alias(libs.plugins.kotlinMultiplatform)
@@ -9,6 +10,7 @@ plugins {
     alias(libs.plugins.composeCompiler)
     alias(libs.plugins.composeHotReload)
     alias(libs.plugins.kotlinxSerialization)
+    alias(libs.plugins.googleServices)
 }
 
 kotlin {
@@ -120,6 +122,10 @@ android {
 
 dependencies {
     debugImplementation(compose.uiTooling)
+
+    // Firebase Cloud Messaging - use specific version for KMM compatibility
+    implementation("com.google.firebase:firebase-messaging-ktx:24.1.0")
+    implementation("com.google.firebase:firebase-bom:34.0.0")
 }
 
 compose.desktop {