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/lib/utils.ts

@@ -0,0 +1,12 @@
+export function formatCurrency(amount: number): string {
+  return new Intl.NumberFormat('es-CL', { style: 'currency', currency: 'CLP', maximumFractionDigits: 0 }).format(amount);
+}
+
+export function formatDate(date: string | Date): string {
+  const d = typeof date === 'string' ? new Date(date) : date;
+  return d.toLocaleDateString('es-CL', { day: '2-digit', month: '2-digit', year: 'numeric' });
+}
+
+export function classNames(...classes: (string | boolean | undefined)[]): string {
+  return classes.filter(Boolean).join(' ');
+}