fix(theme): 用整页快照过渡消除明暗割裂

- 支持 View Transition 时整页旧/新快照交叉淡入 160ms，背景图与底色不再各自过渡
- 不支持或减少动画偏好时回退到原来的颜色过渡

stash-web/src/App.css

@@ -1422,3 +1422,17 @@ html.theme-transition .site-background {
     grid-template-columns: minmax(0, 1fr);
   }
 }
+
+/* 明暗切换：支持的浏览器直接对整页做快照交叉淡入，避免背景图和底色各变各的 */
+::view-transition-old(root),
+::view-transition-new(root) {
+  animation-duration: 160ms;
+  animation-timing-function: var(--su-ease-in-out);
+}
+
+@media (prefers-reduced-motion: reduce) {
+  ::view-transition-old(root),
+  ::view-transition-new(root) {
+    animation: none;
+  }
+}