Refine admin shell to Tabler style

src/TaxBaik.Web/wwwroot/css/admin.css

@@ -345,6 +345,76 @@ html, body {
   min-height: 100vh;
 }
 
+.admin-shell {
+  background:
+    radial-gradient(circle at top left, rgba(37, 99, 235, 0.08), transparent 26%),
+    radial-gradient(circle at top right, rgba(15, 23, 42, 0.04), transparent 22%),
+    linear-gradient(180deg, #F5F8FC 0%, #EDF2F8 100%);
+}
+
+.admin-topbar {
+  backdrop-filter: blur(10px);
+  background: rgba(255, 255, 255, 0.88) !important;
+  border-bottom: 1px solid rgba(214, 223, 232, 0.85);
+  box-shadow: 0 1px 2px rgba(15, 23, 42, 0.04);
+}
+
+.admin-sidebar-card {
+  border-radius: var(--radius-xl);
+  overflow: hidden;
+  box-shadow: var(--shadow-sm);
+}
+
+.admin-sidebar-header {
+  background: linear-gradient(180deg, #FFFFFF 0%, #F8FAFC 100%);
+  border-bottom: 1px solid var(--border-color-light);
+}
+
+.admin-sidebar-body {
+  background: #fff;
+}
+
+.admin-sidebar-card .accordion-button {
+  font-weight: var(--font-weight-semibold);
+  font-size: 0.88rem;
+  color: var(--text-primary);
+  background: #fff;
+}
+
+.admin-sidebar-card .accordion-button:not(.collapsed) {
+  color: var(--primary-dark);
+  background: linear-gradient(90deg, rgba(31, 78, 121, 0.08), rgba(31, 78, 121, 0.03));
+  box-shadow: inset 3px 0 0 var(--primary-color);
+}
+
+.admin-sidebar-card .list-group-item {
+  border: 0;
+  border-radius: 0;
+  padding: 0.85rem 1rem;
+  color: var(--text-primary);
+  background: transparent;
+}
+
+.admin-sidebar-card .list-group-item:hover {
+  background: rgba(31, 78, 121, 0.06);
+  color: var(--primary-dark);
+}
+
+.admin-sidebar-card .list-group-item.active {
+  background: linear-gradient(90deg, rgba(31, 78, 121, 0.14), rgba(31, 78, 121, 0.06));
+  color: var(--primary-dark);
+  font-weight: var(--font-weight-semibold);
+}
+
+.admin-main-surface {
+  border-radius: var(--radius-xl);
+  overflow: hidden;
+}
+
+.admin-content-card .card-body {
+  min-height: calc(100vh - 140px);
+}
+
 .admin-faq-edit-panel {
   max-width: 720px;
 }