feat: Implement Firebase push notifications with support for flavor-specific configurations.

android/app/src/main/AndroidManifest.xml

@@ -2,6 +2,7 @@
     <uses-permission android:name="android.permission.CAMERA"/>
     <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" android:maxSdkVersion="32"/>
     <uses-permission android:name="android.permission.READ_MEDIA_IMAGES"/>
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS"/>
     <application
         android:label="@string/app_name"
         android:name="${applicationName}"
@@ -33,6 +34,10 @@
         <meta-data
             android:name="flutterEmbedding"
             android:value="2" />
+        <!-- FCM default notification channel -->
+        <meta-data
+            android:name="com.google.firebase.messaging.default_notification_channel_id"
+            android:value="high_importance_channel" />
     </application>
     <!-- Required to query activities that can process text, see:
          https://developer.android.com/training/package-visibility and