修复注册按钮内容显示在下半部分的问题

根本原因：
- 按钮使用了 align-items: center，导致内容垂直居中
- 按钮使用了固定高度，没有充满 nav-header 的完整高度

解决方案：
- 改为 align-items: flex-start，让内容从顶部开始排列
- 按钮高度改为 height: 100%，充分填充 nav-header 高度
- 在所有三个移动端断点和桌面端应用这个改变
- 添加小的 padding-top (2-3px) 来微调上方间距

现在注册按钮会完全占满导航栏的高度，图标和文字从顶部开始显示

🤖 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: center !important;
+  align-items: flex-start !important;
   justify-content: center !important;
   gap: 8px !important;
   padding: 0 36px !important;
@@ -159,6 +159,8 @@
   min-width: 120px !important;
   flex-shrink: 0 !important;
   box-shadow: none !important;
+  height: 100% !important;
+  padding-top: 3px !important;
 }
 
 /* 注册按钮文字 - 确保始终在最上层 */
@@ -2551,15 +2553,16 @@ html {
     padding: 0 10px !important;
     font-size: 10px !important;
     min-width: auto !important;
-    height: 46px !important;
+    height: 100% !important;
     line-height: 1 !important;
     display: flex !important;
-    align-items: center !important;
+    align-items: flex-start !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;
   }
 
   #index .no-login .btn.btn-normal#registBtn::before {
@@ -3108,10 +3111,10 @@ html {
     padding: 0 10px !important;
     font-size: 10px !important;
     min-width: auto !important;
-    height: 44px !important;
+    height: 100% !important;
     line-height: 1 !important;
     display: flex !important;
-    align-items: center !important;
+    align-items: flex-start !important;
     justify-content: center !important;
     margin: 0 !important;
     border: none !important;
@@ -3610,10 +3613,10 @@ html {
     padding: 0 8px !important;
     font-size: 9px !important;
     min-width: auto !important;
-    height: 40px !important;
+    height: 100% !important;
     line-height: 1 !important;
     display: flex !important;
-    align-items: center !important;
+    align-items: flex-start !important;
     justify-content: center !important;
     margin: 0 !important;
     border: none !important;