Phase 2 scaffold: mitra online status & pairing logic

Add mitra online/offline status with heartbeat-based auto-offline,
customer-mitra pairing via Valkey pub/sub blast, session management,
and control center dashboard with real-time stats.

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

mitra_app/lib/router.dart

@@ -3,6 +3,7 @@ import 'core/auth/auth_bloc.dart';
 import 'features/auth/screens/login_screen.dart';
 import 'features/auth/screens/otp_screen.dart';
 import 'features/home/home_screen.dart';
+import 'features/chat/screens/active_sessions_screen.dart';
 
 GoRouter buildRouter(AuthBloc authBloc) {
   return GoRouter(
@@ -20,6 +21,7 @@ GoRouter buildRouter(AuthBloc authBloc) {
       GoRoute(path: '/login', builder: (_, __) => const LoginScreen()),
       GoRoute(path: '/otp', builder: (context, state) => OtpScreen(phone: state.extra as String)),
       GoRoute(path: '/home', builder: (_, __) => const HomeScreen()),
+      GoRoute(path: '/sessions', builder: (_, __) => const ActiveSessionsScreen()),
     ],
   );
 }