fix(theme): 背景图不再跟着半程淡出

切深色时立即隐藏背景图，切浅色时等底色过渡完再淡入，避免 65vh 边界出现半黑半白

stash-web/src/App.css

@@ -22,7 +22,7 @@
   height: 65vh;
   background: url('./assets/site-bg.jpg') center top / cover no-repeat;
   pointer-events: none;
-  transition: opacity 160ms var(--su-ease-in-out);
+  transition: opacity 160ms var(--su-ease-in-out) 160ms;
 }
 
 .site-background::after {
@@ -35,9 +35,10 @@
   background: linear-gradient(to bottom, transparent, var(--su-bg-page));
 }
 
-/* 这张背景图是浅色渐变图，深色下留着会糊成一层脏灰，淡出而不是直接消失 */
+/* 这张背景图是浅色渐变图，深色下留着会糊成一层脏灰；切深色时立刻隐藏，避免半黑半白 */
 [data-scheme='dark'] .site-background {
   opacity: 0;
+  transition: opacity 0s;
 }
 
 /* ---------- 页头 ---------- */
@@ -1353,3 +1354,12 @@ html.theme-transition *::after {
     transition: none !important;
   }
 }
+
+/* 背景图不跟着全局过渡：切深色立即消失，切浅色等底色过渡完再淡入 */
+html.theme-transition .site-background {
+  transition: opacity 160ms var(--su-ease-in-out) 160ms !important;
+}
+
+html.theme-transition[data-scheme='dark'] .site-background {
+  transition: opacity 0s !important;
+}