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/ModalConfirmacion.tsx

@@ -0,0 +1,27 @@
+'use client';
+
+interface Props {
+  open: boolean;
+  title: string;
+  message: string;
+  onConfirm: () => void;
+  onCancel: () => void;
+  confirmText?: string;
+}
+
+export default function ModalConfirmacion({ open, title, message, onConfirm, onCancel, confirmText = 'Aceptar' }: Props) {
+  if (!open) return null;
+
+  return (
+    <div className="modal-overlay" onClick={onCancel}>
+      <div className="modal-content" onClick={e => e.stopPropagation()}>
+        <h3>{title}</h3>
+        <p>{message}</p>
+        <div className="modal-actions">
+          <button className="btn btn-secondary" onClick={onCancel}>Cancelar</button>
+          <button className="btn btn-primary" onClick={onConfirm}>{confirmText}</button>
+        </div>
+      </div>
+    </div>
+  );
+}