iter(g51): 新增 backTop 返回顶部按钮

packages/skin/ui/ui.css

@@ -2963,3 +2963,54 @@ button.ua-step-dot:hover {
   .ua-datepicker-day,
   .ua-datepicker-nav { transition: none; }
 }
+.ua-backtop {
+  position: fixed;
+  right: 24px;
+  bottom: 24px;
+  z-index: 60;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 44px;
+  height: 44px;
+  padding: 0;
+  color: var(--ua-color-accent-contrast);
+  background: var(--ua-color-accent);
+  border: none;
+  border-radius: 50%;
+  cursor: pointer;
+  opacity: 0;
+  transform: translateY(8px) scale(0.9);
+  pointer-events: none;
+  transition: opacity 0.2s ease, transform 0.2s ease;
+  box-shadow: 0 8px 20px color-mix(in srgb, var(--ua-color-accent) 35%, transparent);
+}
+.ua-backtop.is-visible {
+  opacity: 1;
+  transform: none;
+  pointer-events: auto;
+}
+.ua-backtop.is-visible:hover {
+  transform: translateY(-2px);
+}
+.ua-backtop:focus-visible {
+  outline: 2px solid var(--ua-color-accent);
+  outline-offset: 3px;
+}
+.ua-backtop-ring {
+  position: absolute;
+  inset: -4px;
+  border-radius: 50%;
+  pointer-events: none;
+  -webkit-mask: radial-gradient(farthest-side, transparent calc(100% - 3px), var(--ua-color-text) calc(100% - 3px));
+  mask: radial-gradient(farthest-side, transparent calc(100% - 3px), var(--ua-color-text) calc(100% - 3px));
+}
+.ua-backtop-icon {
+  font-size: var(--ua-font-size-lg);
+  line-height: 1;
+}
+@media (prefers-reduced-motion: reduce) {
+  .ua-backtop {
+    transition: none;
+  }
+}