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/src/androidMain/AndroidManifest.xml

@@ -3,6 +3,7 @@
 
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.CAMERA" />
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
 
     <queries>
         <intent>
@@ -50,6 +51,20 @@
                 android:name="android.support.FILE_PROVIDER_PATHS"
                 android:resource="@xml/file_paths" />
         </provider>
+
+        <!-- Firebase Cloud Messaging Service -->
+        <service
+            android:name=".MyFirebaseMessagingService"
+            android:exported="false">
+            <intent-filter>
+                <action android:name="com.google.firebase.MESSAGING_EVENT" />
+            </intent-filter>
+        </service>
+
+        <!-- Default notification channel ID -->
+        <meta-data
+            android:name="com.google.firebase.messaging.default_notification_channel_id"
+            android:value="@string/default_notification_channel_id" />
     </application>
 
 </manifest>