chore: update push guide

Author: muhsin-k

contributing-guide/mobile-app/setup-guide.mdx

@@ -113,11 +113,93 @@ npx expo install package-name
 pnpm install package-name
 ```
 
-## Push notification
+## Push notifications
 
-If you are using the community edition of Chatwoot, you can now use the [official mobile app](https://www.chatwoot.com/mobile-apps) with push notifications without any additional configuration. 
+Chatwoot supports mobile push notifications through Firebase Cloud Messaging (FCM). There are two delivery paths depending on your setup:
 
-For more details, please refer to the [push notification documentation](https://www.chatwoot.com/hc/handbook/articles/1687935909-push-notification).
+- **Relay server (default)**: Self-hosted instances without Firebase credentials route notifications through the Chatwoot relay server, which forwards them to the **official Chatwoot mobile app**.
+- **Direct FCM**: Instances with Firebase configured send notifications directly to your **custom-built mobile app**.
+
+<Note>
+You must use either the official apps for both platforms or custom builds for both — mixing is not supported. For more details, refer to the [push notification documentation](https://www.chatwoot.com/hc/handbook/articles/1687935909-push-notification).
+</Note>
+
+### Using the official mobile app
+
+If you are using the community edition of Chatwoot with the [official mobile app](https://www.chatwoot.com/mobile-apps), push notifications work out of the box with no additional configuration. The relay server is enabled by default (`ENABLE_PUSH_RELAY_SERVER=true`).
+
+### Setting up Firebase for custom builds
+
+If you are building a custom-branded mobile app, you need to configure Firebase on both the mobile app and the Chatwoot server.
+
+#### Step 1: Create a Firebase project
+
+1. Go to the [Firebase Console](https://console.firebase.google.com/) and create a new project (or use an existing one).
+2. Register your Android app with your package name (e.g., `com.yourcompany.app`).
+3. Register your iOS app with your bundle identifier (e.g., `com.yourcompany.app`).
+
+#### Step 2: Download Firebase config files
+
+Download the platform-specific configuration files from your Firebase project settings:
+
+<Tabs>
+<Tab title="Android">
+1. In **Project Settings > General**, find your Android app and click **Download google-services.json**.
+2. Place the file in the root of the mobile app repository.
+</Tab>
+<Tab title="iOS">
+1. In **Project Settings > General**, find your iOS app and click **Download GoogleService-Info.plist**.
+2. Place the file in the root of the mobile app repository.
+</Tab>
+</Tabs>
+
+#### Step 3: Configure mobile app environment variables
+
+Update your `.env` file to point to the Firebase config files:
+
+```bash
+EXPO_PUBLIC_ANDROID_GOOGLE_SERVICES_FILE=./google-services.json
+EXPO_PUBLIC_IOS_GOOGLE_SERVICES_FILE=./GoogleService-Info.plist
+```
+
+Then regenerate the native code so the new config is picked up:
+
+```bash
+pnpm generate
+```
+
+#### Step 4: Generate a Firebase service account
+
+The Chatwoot server needs a service account to send push notifications via the FCM v1 API.
+
+1. In the Firebase Console, go to **Project Settings > Service accounts**.
+2. Click **Generate new private key** to download a JSON credentials file.
+
+<Warning>
+Keep this file secure. It grants access to send push notifications on behalf of your Firebase project.
+</Warning>
+
+#### Step 5: Configure the Chatwoot server
+
+In your Chatwoot installation, navigate to **Super Admin > App Config** and set the following:
+
+| Config key              | Value                                                     |
+| ----------------------- | --------------------------------------------------------- |
+| `FIREBASE_PROJECT_ID`   | Your Firebase project ID (e.g., `my-project-12345`)       |
+| `FIREBASE_CREDENTIALS`  | The full contents of the service account JSON file         |
+
+Once both values are set, the server will send push notifications directly to FCM instead of using the relay server.
+
+#### Verifying the setup
+
+1. Build and install the custom app on a device (`pnpm run:ios` or `pnpm run:android`).
+2. Log in to your Chatwoot instance from the mobile app.
+3. From another browser session, send a message to a conversation assigned to the logged-in agent.
+4. The device should receive a push notification.
+
+<Note>
+Push notifications do not work on iOS simulators. You must use a physical device to test.
+</Note>
 
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