修复注册按钮不显示的问题

- 改为 align-items: center 垂直居中对齐
- 移除 padding-top 防止按钮溢出
- 添加 box-sizing: border-box 确保 height 100% 包含 padding
- 应用于所有断点：桌面端、767px、575px、360px

问题原因：height: 100% + padding-top 导致按钮总高度超出容器

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

css/index.css

@@ -140,7 +140,7 @@
 /* 注册按钮 - 渐变背景样式，撑满高度 */
 #index .no-login .btn.btn-normal#registBtn {
   display: flex !important;
-  align-items: flex-start !important;
+  align-items: center !important;
   justify-content: center !important;
   gap: 8px !important;
   padding: 0 36px !important;
@@ -160,7 +160,7 @@
   flex-shrink: 0 !important;
   box-shadow: none !important;
   height: 100% !important;
-  padding-top: 3px !important;
+  box-sizing: border-box !important;
 }
 
 /* 注册按钮文字 - 确保始终在最上层 */
@@ -2556,13 +2556,13 @@ html {
     height: 100% !important;
     line-height: 1 !important;
     display: flex !important;
-    align-items: flex-start !important;
+    align-items: center !important;
     justify-content: center !important;
     margin: 0 !important;
     border: none !important;
     background: linear-gradient(135deg, #38BDF8 0%, #6366F1 100%) !important;
     flex-shrink: 0 !important;
-    padding-top: 2px !important;
+    box-sizing: border-box !important;
   }
 
   #index .no-login .btn.btn-normal#registBtn::before {
@@ -3114,12 +3114,13 @@ html {
     height: 100% !important;
     line-height: 1 !important;
     display: flex !important;
-    align-items: flex-start !important;
+    align-items: center !important;
     justify-content: center !important;
     margin: 0 !important;
     border: none !important;
     background: linear-gradient(135deg, #38BDF8 0%, #6366F1 100%) !important;
     flex-shrink: 0 !important;
+    box-sizing: border-box !important;
   }
 
   #index .no-login .btn.btn-normal#registBtn::before {
@@ -3616,12 +3617,13 @@ html {
     height: 100% !important;
     line-height: 1 !important;
     display: flex !important;
-    align-items: flex-start !important;
+    align-items: center !important;
     justify-content: center !important;
     margin: 0 !important;
     border: none !important;
     background: linear-gradient(135deg, #38BDF8 0%, #6366F1 100%) !important;
     flex-shrink: 0 !important;
+    box-sizing: border-box !important;
   }
 
   #index .no-login .btn.btn-normal#registBtn::before {