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>

client_app/lib/features/chat/screens/no_bestie_screen.dart

@@ -0,0 +1,39 @@
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+
+class NoBestieScreen extends StatelessWidget {
+  const NoBestieScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: Center(
+        child: Padding(
+          padding: const EdgeInsets.all(32),
+          child: Column(
+            mainAxisAlignment: MainAxisAlignment.center,
+            children: [
+              const Icon(Icons.sentiment_dissatisfied, size: 80, color: Colors.orange),
+              const SizedBox(height: 24),
+              const Text(
+                'Bestie belum tersedia',
+                style: TextStyle(fontSize: 24, fontWeight: FontWeight.bold),
+              ),
+              const SizedBox(height: 8),
+              const Text(
+                'Maaf, semua Bestie sedang sibuk. Coba lagi nanti ya.',
+                textAlign: TextAlign.center,
+                style: TextStyle(fontSize: 16, color: Colors.grey),
+              ),
+              const SizedBox(height: 48),
+              ElevatedButton(
+                onPressed: () => context.go('/home'),
+                child: const Text('Kembali'),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}