Phase 3.1 WS2: FCM fallback Flutter + CC, unread badges, dynamic ping

- Control center: add mitra ping config UI (require ping toggle + interval)
- Mitra app StatusNotifier: honor require_ping and ping_interval_seconds
  from API; skip heartbeat when ping not required
- Both apps: update notification services for FCM deep-linking
  - mitra_app: handle chat_request (open_accept), session_closing
  - client_app: handle session_closing, paired
- Unread badge providers:
  - mitra_app: UnreadSessions provider (polls active-with-unread, badge
    on active sessions button)
  - client_app: UnreadCount provider (polls active-with-unread, badge
    on _ActiveSessionCard)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mitra_app/lib/core/status/status_notifier.dart

@@ -30,6 +30,8 @@ class StatusErrorData extends OnlineStatusData {
 @Riverpod(keepAlive: true)
 class OnlineStatus extends _$OnlineStatus {
   Timer? _heartbeatTimer;
+  bool _requirePing = true;
+  int _pingIntervalSeconds = 15;
 
   @override
   OnlineStatusData build() => const StatusInitialData();
@@ -38,6 +40,8 @@ class OnlineStatus extends _$OnlineStatus {
     try {
       final response = await ref.read(apiClientProvider).get('/api/mitra/status');
       final data = response['data'] as Map<String, dynamic>;
+      _requirePing = data['require_ping'] as bool? ?? true;
+      _pingIntervalSeconds = data['ping_interval_seconds'] as int? ?? 15;
       state = StatusLoadedData(isOnline: data['is_online'] as bool);
     } catch (e) {
       state = const StatusLoadedData(isOnline: false);
@@ -48,7 +52,7 @@ class OnlineStatus extends _$OnlineStatus {
     state = const StatusLoadingData();
     try {
       await ref.read(apiClientProvider).post('/api/mitra/status/online');
-      _startHeartbeat();
+      if (_requirePing) _startHeartbeat();
       state = const StatusLoadedData(isOnline: true);
     } catch (e) {
       state = const StatusErrorData('Gagal mengubah status. Coba lagi.');
@@ -67,11 +71,11 @@ class OnlineStatus extends _$OnlineStatus {
   }
 
   void onAppPaused() {
-    _stopHeartbeat();
+    if (_requirePing) _stopHeartbeat();
   }
 
   void onAppResumed() {
-    if (state is StatusLoadedData && (state as StatusLoadedData).isOnline) {
+    if (_requirePing && state is StatusLoadedData && (state as StatusLoadedData).isOnline) {
       _startHeartbeat();
     }
     load();
@@ -79,7 +83,7 @@ class OnlineStatus extends _$OnlineStatus {
 
   void _startHeartbeat() {
     _stopHeartbeat();
-    _heartbeatTimer = Timer.periodic(const Duration(seconds: 15), (_) {
+    _heartbeatTimer = Timer.periodic(Duration(seconds: _pingIntervalSeconds), (_) {
       _heartbeatTick();
     });
   }