style: add Tabler navbar-vertical CSS for admin sidebar with proper typography and colors

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

@@ -1846,6 +1846,150 @@ textarea:focus-visible {
     min-width: 100px;
     font-size: var(--font-size-xs);
   }
+}
+
+/* ============================================================================
+   Tabler Navbar Vertical (Admin Sidebar)
+   ============================================================================ */
+
+.admin-sidebar-nav {
+  background: var(--bg-primary);
+  border-right: 1px solid var(--border-color-light);
+  padding: 0;
+  min-height: 100%;
+  flex-direction: column;
+}
+
+.admin-sidebar-nav .navbar-brand {
+  padding: var(--space-3) var(--space-4);
+  border-bottom: 1px solid var(--border-color-light);
+}
+
+.admin-sidebar-nav .navbar-nav {
+  flex-direction: column;
+  padding: var(--space-4) 0;
+}
+
+.admin-sidebar-nav .nav-item {
+  margin: 0;
+}
+
+.admin-sidebar-nav .nav-link {
+  display: flex;
+  align-items: center;
+  gap: var(--space-3);
+  padding: var(--space-3) var(--space-4);
+  color: var(--text-primary);
+  text-decoration: none;
+  font-size: var(--font-size-sm);
+  font-weight: var(--font-weight-regular);
+  transition: all var(--transition-base);
+  position: relative;
+  border-left: 3px solid transparent;
+  margin: 0 var(--space-2);
+  border-radius: var(--radius-md);
+}
+
+.admin-sidebar-nav .nav-link:hover {
+  background-color: var(--primary-light);
+  color: var(--primary-dark);
+}
+
+.admin-sidebar-nav .nav-link.active {
+  background-color: var(--primary-light);
+  color: var(--primary-dark);
+  font-weight: var(--font-weight-semibold);
+  border-left-color: var(--primary-color);
+}
+
+.admin-sidebar-nav .nav-link-icon {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 20px;
+  height: 20px;
+  flex-shrink: 0;
+}
+
+.admin-sidebar-nav .nav-link-icon svg {
+  width: 20px;
+  height: 20px;
+  stroke: currentColor;
+}
+
+.admin-sidebar-nav .nav-link-title {
+  flex: 1;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.admin-sidebar-nav .dropdown-toggle::after {
+  content: '';
+  display: inline-block;
+  width: 0.3em;
+  height: 0.3em;
+  border: solid currentColor;
+  border-width: 0.15em 0.15em 0 0;
+  transform: rotate(45deg);
+  margin-left: auto;
+  transition: transform var(--transition-base);
+}
+
+.admin-sidebar-nav .dropdown-toggle:not(.collapsed)::after {
+  transform: rotate(-135deg);
+}
+
+.admin-sidebar-nav .dropdown-menu {
+  border: none;
+  background: var(--bg-secondary);
+  padding: var(--space-2) 0;
+  margin: 0;
+  border-radius: 0;
+  box-shadow: inset 2px 0 0 var(--border-color-light);
+}
+
+.admin-sidebar-nav .dropdown-item {
+  padding: var(--space-3) var(--space-4) var(--space-3) calc(var(--space-4) + 20px + var(--space-3));
+  color: var(--text-primary);
+  font-size: var(--font-size-sm);
+  transition: all var(--transition-base);
+  text-decoration: none;
+}
+
+.admin-sidebar-nav .dropdown-item:hover {
+  background-color: var(--primary-light);
+  color: var(--primary-dark);
+}
+
+.admin-sidebar-nav .dropdown-item.active {
+  background-color: var(--primary-light);
+  color: var(--primary-dark);
+  font-weight: var(--font-weight-semibold);
+}
+
+/* Responsive Navbar Toggler */
+.admin-sidebar-nav .navbar-toggler {
+  display: none;
+  border: none;
+  padding: var(--space-3) var(--space-4);
+  color: var(--text-primary);
+}
+
+@media (max-width: 991px) {
+  .admin-sidebar-nav .navbar-toggler {
+    display: block;
+  }
+
+  .admin-sidebar-nav {
+    position: sticky;
+    top: 0;
+  }
+
+  .admin-sidebar-nav .navbar-collapse {
+    max-height: calc(100vh - 150px);
+    overflow-y: auto;
+  }
 
   .admin-drawer-footer {
     display: none;