style: 优化登录框垂直位置

将登录框顶部留白从 18vh 增加到 25vh
让登录框从页面约 1/4 处开始，视觉重心更舒适

修改：
- .content padding: 18vh → 25vh
- 对应更新 padding-top 计算

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mini-program/src/pages/login/index.vue

@@ -200,9 +200,9 @@ const handleLogin = async () => {
   display: flex;
   align-items: flex-start;
   justify-content: center;
-  padding: 18vh 40rpx;
-  padding-top: calc(18vh + constant(safe-area-inset-top));
-  padding-top: calc(18vh + env(safe-area-inset-top));
+  padding: 25vh 40rpx;
+  padding-top: calc(25vh + constant(safe-area-inset-top));
+  padding-top: calc(25vh + env(safe-area-inset-top));
 }
 
 .login-card {