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/chat/unread_notifier.dart

@@ -0,0 +1,54 @@
+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 UnreadSessions extends _$UnreadSessions {
+  Timer? _pollTimer;
+
+  @override
+  Map<String, int> build() {
+    _startPolling();
+    ref.onDispose(_stopPolling);
+    return {};
+  }
+
+  void _startPolling() {
+    _stopPolling();
+    _fetchUnreadCounts();
+    _pollTimer = Timer.periodic(const Duration(seconds: 15), (_) {
+      _fetchUnreadCounts();
+    });
+  }
+
+  void _stopPolling() {
+    _pollTimer?.cancel();
+    _pollTimer = null;
+  }
+
+  Future<void> _fetchUnreadCounts() async {
+    try {
+      final apiClient = ref.read(apiClientProvider);
+      final response = await apiClient.get('/api/mitra/chat-requests/sessions/active-with-unread');
+      final sessions = response['data'] as List<dynamic>;
+      final counts = <String, int>{};
+      for (final s in sessions) {
+        final id = s['id'] as String;
+        final count = s['unread_count'] as int? ?? 0;
+        if (count > 0) counts[id] = count;
+      }
+      state = counts;
+    } catch (_) {}
+  }
+
+  int get totalUnread => state.values.fold(0, (a, b) => a + b);
+
+  void markSessionRead(String sessionId) {
+    state = {...state}..remove(sessionId);
+  }
+
+  void refresh() => _fetchUnreadCounts();
+}

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

@@ -0,0 +1,26 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'unread_notifier.dart';
+
+// **************************************************************************
+// RiverpodGenerator
+// **************************************************************************
+
+String _$unreadSessionsHash() => r'd2ff837f1e781e6aa624b3d3ca2befb0d1d258e8';
+
+/// See also [UnreadSessions].
+@ProviderFor(UnreadSessions)
+final unreadSessionsProvider =
+    NotifierProvider<UnreadSessions, Map<String, int>>.internal(
+  UnreadSessions.new,
+  name: r'unreadSessionsProvider',
+  debugGetCreateSourceHash: const bool.fromEnvironment('dart.vm.product')
+      ? null
+      : _$unreadSessionsHash,
+  dependencies: null,
+  allTransitiveDependencies: null,
+);
+
+typedef _$UnreadSessions = Notifier<Map<String, 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