Client: mirror branded notification sound to customer app

The customer app now uses the same halobestie_notif.ogg as the mitra
app (shipped in the previous commit). Channel ID unified across both
apps so backend FCM stops branching per recipient.

- client_app: same channel bump (chat_messages → halobestie_chat_v1)
  + RawResourceAndroidNotificationSound binding, both at channel-
  create time and per-notification details. .ogg copied to
  client_app/android/app/src/main/res/raw/halobestie_notif.ogg
  (same 32 KB asset, identical file).
- Backend: drop the per-recipientType channel ID branch; everyone
  targets halobestie_chat_v1 now.

Verified on emulator-5554 (customer): dumpsys shows the channel
bound to android.resource://com.mybestie/raw/halobestie_notif.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client_app/lib/core/notifications/notification_service.dart

@@ -8,11 +8,17 @@ class NotificationService {
   static final _localNotifications = FlutterLocalNotificationsPlugin();
   static GoRouter? _router;
 
+  // Channel ID bumped (`chat_messages` → `halobestie_chat_v1`) when the
+  // branded notification sound was introduced. Android binds sound to a
+  // channel at create time on API 26+, so an existing channel can't pick
+  // up a new sound — a fresh ID is the only way. Backend FCM payloads
+  // target the same ID — see backend/src/services/notification.service.js.
   static const _channel = AndroidNotificationChannel(
-    'chat_messages',
-    'Chat Messages',
-    description: 'Notifications for incoming chat messages',
+    'halobestie_chat_v1',
+    'Chat HaloBestie',
+    description: 'Notifications for incoming chat messages and pairing requests',
     importance: Importance.high,
+    sound: RawResourceAndroidNotificationSound('halobestie_notif'),
   );
 
   static Future<void> initialize(GoRouter router) async {
@@ -60,6 +66,9 @@ class NotificationService {
           channelDescription: _channel.description,
           importance: Importance.high,
           priority: Priority.high,
+          // API 26+ ignores this in favor of the channel's sound; included
+          // for the API 24/25 path where channels don't exist yet.
+          sound: const RawResourceAndroidNotificationSound('halobestie_notif'),
         ),
         iOS: const DarwinNotificationDetails(
           presentAlert: true,