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>

client_app/lib/core/chat/unread_notifier.dart

@@ -0,0 +1,50 @@
+import 'dart:async';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:riverpod_annotation/riverpod_annotation.dart';
+import '../api/api_client_provider.dart';
+
+part 'unread_notifier.g.dart';
+
+@Riverpod(keepAlive: true)
+class UnreadCount extends _$UnreadCount {
+  Timer? _pollTimer;
+
+  @override
+  int build() {
+    _startPolling();
+    ref.onDispose(_stopPolling);
+    return 0;
+  }
+
+  void _startPolling() {
+    _stopPolling();
+    _fetchUnreadCount();
+    _pollTimer = Timer.periodic(const Duration(seconds: 15), (_) {
+      _fetchUnreadCount();
+    });
+  }
+
+  void _stopPolling() {
+    _pollTimer?.cancel();
+    _pollTimer = null;
+  }
+
+  Future<void> _fetchUnreadCount() async {
+    try {
+      final apiClient = ref.read(apiClientProvider);
+      final response = await apiClient.get('/api/client/chat/session/active-with-unread');
+      final data = response['data'];
+      if (data is Map<String, dynamic>) {
+        state = data['unread_count'] as int? ?? 0;
+      } else {
+        state = 0;
+      }
+    } catch (_) {}
+  }
+
+  void markRead() {
+    state = 0;
+  }
+
+  void refresh() => _fetchUnreadCount();
+}

client_app/lib/core/chat/unread_notifier.g.dart

@@ -0,0 +1,24 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'unread_notifier.dart';
+
+// **************************************************************************
+// RiverpodGenerator
+// **************************************************************************
+
+String _$unreadCountHash() => r'6a0b31b86ae616177f54346392d9675f916a7bec';
+
+/// See also [UnreadCount].
+@ProviderFor(UnreadCount)
+final unreadCountProvider = NotifierProvider<UnreadCount, int>.internal(
+  UnreadCount.new,
+  name: r'unreadCountProvider',
+  debugGetCreateSourceHash:
+      const bool.fromEnvironment('dart.vm.product') ? null : _$unreadCountHash,
+  dependencies: null,
+  allTransitiveDependencies: null,
+);
+
+typedef _$UnreadCount = Notifier<int>;
+// ignore_for_file: type=lint
+// ignore_for_file: subtype_of_sealed_class, invalid_use_of_internal_member, invalid_use_of_visible_for_testing_member, deprecated_member_use_from_same_package

client_app/lib/core/notifications/notification_service.dart

@@ -84,12 +84,17 @@ class NotificationService {
   }
 
   static void _navigateFromMessage(Map<String, dynamic> data) {
+    if (_router == null) return;
     final sessionId = data['session_id'] as String?;
-    if (sessionId == null || _router == null) return;
-
     final type = data['type'] as String?;
-    if (type == 'chat_message' || type == 'chat_request') {
-      _router!.push('/chat/session/$sessionId');
+
+    if (type == 'session_closing' || type == 'session_expired') {
+      // Navigate to the chat session — closure UI will show
+      if (sessionId != null) {
+        _router!.push('/chat/session/$sessionId', extra: 'Bestie');
+      }
+    } else if ((type == 'chat_message' || type == 'paired') && sessionId != null) {
+      _router!.push('/chat/session/$sessionId', extra: 'Bestie');
     }
   }
 }