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/Sidebar.tsx

@@ -0,0 +1,43 @@
+'use client';
+
+import Link from 'next/link';
+import { usePathname } from 'next/navigation';
+import { useState } from 'react';
+
+const menuItems = [
+  { href: '/dashboard', icon: 'fas fa-home', label: 'Dashboard' },
+  { href: '/leads', icon: 'fas fa-users', label: 'Leads' },
+  { href: '/cotizaciones', icon: 'fas fa-file-invoice-dollar', label: 'Cotizaciones' },
+  { href: '/cursos', icon: 'fas fa-book', label: 'Cursos' },
+  { href: '/alumnos', icon: 'fas fa-user-graduate', label: 'Alumnos' },
+  { href: '/arqueo', icon: 'fas fa-cash-register', label: 'Arqueo' },
+  { href: '/empresas', icon: 'fas fa-building', label: 'Empresas' },
+  { href: '/reportes/ventas', icon: 'fas fa-chart-bar', label: 'Reportes' },
+  { href: '/autorizador', icon: 'fas fa-check-double', label: 'Autorizador' },
+  { href: '/contacto', icon: 'fas fa-envelope', label: 'Contacto' },
+];
+
+export default function Sidebar() {
+  const pathname = usePathname();
+  const [collapsed, setCollapsed] = useState(false);
+
+  return (
+    <div className={`sidebar ${collapsed ? 'collapsed' : ''}`}>
+      <div className="logo-details">
+        <img className="icon" src="/img/favicon.png" width="60" alt="SAM" />
+        <div className="logo_name">SAM</div>
+        <i className="bx bx-menu" id="btn" onClick={() => setCollapsed(!collapsed)}></i>
+      </div>
+      <ul className="nav-list">
+        {menuItems.map(item => (
+          <li key={item.href} className={pathname.startsWith(item.href) ? 'active' : ''}>
+            <Link href={item.href}>
+              <i className={item.icon}></i>
+              <span className="links_name">{item.label}</span>
+            </Link>
+          </li>
+        ))}
+      </ul>
+    </div>
+  );
+}