fix: 修复登录页面按钮溢出问题（方案 A）

修改内容：
- .login-card 左右内边距 40rpx → 32rpx
- .login-card 添加 box-sizing: border-box
- .btn-primary 添加 padding: 0 和 box-sizing: border-box
- .code-row 添加 box-sizing: border-box

确保所有容器和按钮在小屏幕上不溢出

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

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

@@ -212,7 +212,8 @@ const handleLogin = async () => {
   backdrop-filter: blur(40rpx);
   border: 1px solid rgba(168, 85, 247, 0.15);
   border-radius: 48rpx;
-  padding: 48rpx 40rpx;
+  padding: 48rpx 32rpx;
+  box-sizing: border-box;
 }
 
 .header {
@@ -275,6 +276,7 @@ const handleLogin = async () => {
   gap: 16rpx;
   align-items: stretch;
   width: 100%;
+  box-sizing: border-box;
 }
 
 .code-input {
@@ -329,6 +331,8 @@ const handleLogin = async () => {
   border: none;
   box-shadow: 0 8rpx 32rpx rgba(168, 85, 247, 0.3);
   margin-bottom: 24rpx;
+  padding: 0;
+  box-sizing: border-box;
 }
 
 .btn-primary.disabled {