feat: fase 3 - frontend next.js setup

- Next.js 15 + TypeScript + Bootstrap 5 + FontAwesome
- Layout replica SAM.Master (sidebar collapsable + header)
- Sidebar, Header, LoadingSpinner, ModalConfirmacion components
- Login page with RUT validation
- Dashboard page with summary cards
- AuthContext + JWT auth with cookies
- api.ts centralized HTTP client
- useInactividad hook (30min timeout)
- validarRut.ts + utils.ts migrated
- Original CSS + images copied from project
- Dockerfile multi-stage build

frontend/src/components/Header.tsx

@@ -0,0 +1,30 @@
+'use client';
+
+import { useAuth } from '@/hooks/useAuth';
+import { useRouter } from 'next/navigation';
+
+export default function Header() {
+  const { user, logout } = useAuth();
+  const router = useRouter();
+
+  const handleLogout = () => {
+    logout();
+    router.push('/login');
+  };
+
+  return (
+    <header>
+      <div className="header-content">
+        <span className="badge">Menu de Aplicaciones</span>
+        <div className="user-info">
+          <span className="user-name">
+            <i className="fas fa-user"></i> {user?.nombre || 'Usuario'}
+          </span>
+          <button className="btn-logout" onClick={handleLogout}>
+            <i className="fas fa-sign-out-alt"></i> Cerrar Sesión
+          </button>
+        </div>
+      </div>
+    </header>
+  );
+}