Mitra: branded notification sound (halobestie_notif.ogg)

Drops the system notification ding for incoming mitra FCM (Curhat Baru,
Permintaan Perpanjang) and plays the HaloBestie audio mark instead.

Source: a 2.8s mono AAC inside a 3GPP container the user supplied;
converted to 32 KB OGG (Vorbis q5) for Android since the channel-sound
API needs `res/raw/<name>.<ext>` and OGG is the smallest universally
supported short-sound format on Android 5+.

- mitra_app: bump notification channel ID from `chat_messages` to
  `halobestie_chat_v1` (Android binds channel sound at create time
  on API 26+, so existing installs with the old channel need a fresh
  ID to pick up the new sound — can't mutate in place). Bind
  RawResourceAndroidNotificationSound('halobestie_notif') at both
  channel-create time and per-notification details (latter covers
  API 24/25 where channels don't exist).
- Backend: branch FCM `android.notification.channelId` by recipient
  type — mitras → `halobestie_chat_v1`, customers → `chat_messages`
  (unchanged). Customer app keeps system sound until/unless we ship
  a customer-side sound too.

Verified on emulator-5556 via `adb shell dumpsys notification` — the
new channel resolves to
`android.resource://com.mybestie.mitra/raw/halobestie_notif`. The OGG
ships inside the APK (32092 bytes, confirmed via `unzip -l`).

Follow-up (iOS): bundle the same sound as `.caf` under ios/Runner +
register as a Runner-target resource in pbxproj + reference filename
in the APS payload. Deferred until iOS testing comes back into scope.

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

mitra_app/lib/core/notifications/notification_service.dart

@@ -12,11 +12,18 @@ class NotificationService {
   /// Set this from the app to bridge notifications → Riverpod state.
   static void Function(String sessionId)? onChatRequestTapped;
 
+  // Channel ID is bumped (`chat_messages` → `halobestie_chat_v1`) because
+  // Android binds notification sound at channel-creation time on API 26+. An
+  // existing `chat_messages` channel still routes through whatever sound was
+  // set when it was first created (system default), so we mint a fresh ID
+  // for the branded sound. 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 extension requests',
     importance: Importance.high,
+    sound: RawResourceAndroidNotificationSound('halobestie_notif'),
   );
 
   static Future<void> initialize(GoRouter router) async {
@@ -64,6 +71,9 @@ 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,
@@ -106,6 +116,7 @@ class NotificationService {
           priority: Priority.high,
           playSound: true,
           enableVibration: true,
+          sound: const RawResourceAndroidNotificationSound('halobestie_notif'),
         ),
         iOS: const DarwinNotificationDetails(
           presentAlert: true,