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>

control_center/src/components/Layout.jsx

@@ -9,7 +9,9 @@ export default function Layout() {
       <nav style={{ width: 220, borderRight: '1px solid #eee', padding: 16 }}>
         <h2>Control Center</h2>
         <ul style={{ listStyle: 'none', padding: 0 }}>
+          <li><NavLink to="/dashboard">Dashboard</NavLink></li>
           <li><NavLink to="/mitras">Mitra</NavLink></li>
+          <li><NavLink to="/sessions">Sesi</NavLink></li>
           <li><NavLink to="/users">Users</NavLink></li>
           <li><NavLink to="/settings">Settings</NavLink></li>
         </ul>