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/public/css/ichnPaginas.css

@@ -0,0 +1,80 @@
+.div_1 {
+    height: 80px;
+    background-color: #D1D1D1;
+    font-size: 18px;
+}
+
+.div_2 {
+    min-height: 300px;
+}
+
+label .prueba {
+    cursor: pointer;
+}
+
+.nav-tabs .nav-item.show .nav-link, .nav-tabs .nav-link.active {
+    color: #ffff;
+    background-color: #22376c;
+    border-color: #dee2e6 #dee2e6 #fff;
+}
+
+.nav-tabs .nav-item.show .nav-link, .nav-tabs .nav-link {
+    color: #22376c;
+}
+
+.spinner, .spinner:after {
+    width: 90px;
+    height: 90px;
+    position: fixed;
+    top: 90%;
+    left: 50%;
+    margin-top: -32px;
+    margin-left: -32px;
+    border-radius: 50%;
+    z-index: 2
+}
+
+.spinner {
+    background-color: transparent;
+    border-top: 10px solid rgb(0, 40, 93);
+    border-right: 10px solid rgb(0, 40, 93);
+    border-bottom: 10px solid rgb(0, 40, 93);
+    border-left: 10px solid rgba(0, 40, 93,.2);
+    transform: translateZ(0);
+    animation-iteration-count: infinite;
+    animation-timing-function: linear;
+    animation-duration: 1.5s;
+    animation-name: spinner-loading
+}
+
+.menuIchnHover {
+    background-color: #E8E8E8;
+}
+
+    .menuIchnHover a {
+        color: #212529;
+    }
+
+    .menuIchnHover:hover {
+        background-color: #00285d;
+        color: #fff;
+        border-radius: 5px
+    }
+
+
+        .menuIchnHover:hover a {
+            background-color: #00285d;
+            color: #fff;
+            border-radius: 5px
+        }
+
+
+@keyframes spinner-loading {
+    0% {
+        transform: rotate(0deg)
+    }
+
+    to {
+        transform: rotate(1turn)
+    }
+}