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>

build.gradle.kts

@@ -7,4 +7,5 @@ plugins {
     alias(libs.plugins.composeMultiplatform) apply false
     alias(libs.plugins.composeCompiler) apply false
     alias(libs.plugins.kotlinMultiplatform) apply false
+    alias(libs.plugins.googleServices) apply false
 }