iter: d01 新增下拉菜单 menu 组件（弹层/键盘导航/reduced-motion）

packages/skin/ui/ui.css

@@ -308,6 +308,82 @@
   background: color-mix(in srgb, var(--ua-color-accent) 14%, transparent);
 }
 
+/* —— 下拉菜单 —— */
+.ua-menu {
+  position: relative;
+  display: inline-block;
+}
+.ua-menu-trigger {
+  gap: 6px;
+}
+.ua-menu-caret {
+  font-size: 10px;
+  opacity: 0.7;
+  transition: transform 0.2s;
+}
+.ua-menu.ua-open .ua-menu-caret {
+  transform: rotate(180deg);
+}
+.ua-menu-pop {
+  position: absolute;
+  z-index: 20;
+  top: calc(100% + 4px);
+  left: 0;
+  min-width: 160px;
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+  padding: 4px;
+  border: 1px solid var(--ua-color-border);
+  border-radius: var(--ua-radius-md);
+  background: var(--ua-color-surface);
+  box-shadow: 0 8px 24px rgba(0, 0, 0, 0.14);
+  opacity: 0;
+  transform: translateY(-4px);
+  visibility: hidden;
+  transition: opacity 0.15s ease, transform 0.15s ease, visibility 0.15s;
+}
+.ua-menu-pop.is-end {
+  left: auto;
+  right: 0;
+}
+.ua-menu-pop.ua-open {
+  opacity: 1;
+  transform: none;
+  visibility: visible;
+}
+.ua-menu-item {
+  display: block;
+  width: 100%;
+  text-align: left;
+  padding: 6px 10px;
+  border: none;
+  border-radius: var(--ua-radius-sm);
+  background: none;
+  color: var(--ua-color-text);
+  font-size: var(--ua-font-size-md);
+  cursor: pointer;
+  transition: background 0.12s;
+}
+.ua-menu-item:hover:not(:disabled) {
+  background: color-mix(in srgb, var(--ua-color-accent) 8%, transparent);
+}
+.ua-menu-item.is-danger {
+  color: var(--ua-color-danger);
+}
+.ua-menu-item.is-danger:hover:not(:disabled) {
+  background: color-mix(in srgb, var(--ua-color-danger) 10%, transparent);
+}
+.ua-menu-item:disabled {
+  opacity: 0.45;
+  cursor: not-allowed;
+}
+.ua-menu-divider {
+  height: 1px;
+  margin: 4px 2px;
+  background: var(--ua-color-border);
+}
+
 /* —— 多行输入 —— */
 .ua-textarea {
   width: 100%;
@@ -795,6 +871,9 @@
   .ua-select-arrow,
   .ua-select-pop,
   .ua-option,
+  .ua-menu-pop,
+  .ua-menu-caret,
+  .ua-menu-item,
   .ua-switch-track,
   .ua-switch-thumb,
   .ua-tab,