feat: 切深色模式时高亮/卡片元素加颜色过渡动画

.rank-item / .period-tab / .theme-item / .plan-item 加
transition: background-color 0.3s ease + border-color 0.3s ease。
切深色时不再瞬间切换,有 300ms 流动感。

pages/settings/settings.wxss

@@ -22,7 +22,7 @@
   align-items: center;
   padding: 22rpx 0;
   border-bottom: 1rpx solid var(--border);
-  transition: background 0.2s ease;
+  transition: background 0.2s ease, background-color 0.3s ease, border-color 0.3s ease;
 }
 
 .theme-item:last-child { border-bottom: none; }
@@ -73,6 +73,7 @@
   align-items: center;
   padding: 24rpx 0;
   border-bottom: 1rpx solid var(--border);
+  transition: background-color 0.3s ease, border-color 0.3s ease;
 }
 
 .plan-item:last-child { border-bottom: none; }