feat: 优化移动端顶部导航栏样式和布局

frontend/src/App.vue

@@ -3,11 +3,11 @@
     <!-- 移动端顶部导航栏 -->
     <el-header v-if="isMobile" class="mobile-header">
       <div class="mobile-header-content">
-        <el-button type="primary" link @click="toggleSidebar">
+        <el-button type="primary" link @click="toggleSidebar" class="header-btn">
           <el-icon :size="24"><Fold /></el-icon>
         </el-button>
-        <span v-if="!isMobile" class="mobile-title">投标信息一览</span>
-        <el-button type="primary" link @click="handleLogout" v-if="currentUser">
+        <span v-if="isMobile" class="mobile-title">投标信息一览</span>
+        <el-button type="primary" link @click="handleLogout" v-if="currentUser" class="header-btn">
           <el-icon :size="24"><SwitchButton /></el-icon>
         </el-button>
       </div>
@@ -385,12 +385,21 @@ onUnmounted(() => {
   flex-direction: column;
 }
 
+.is-mobile .el-main {
+  padding-top: 60px;
+}
+
 .mobile-header {
+  position: fixed;
+  top: 0;
+  left: 0;
+  right: 0;
   background-color: #434a50;
   color: white;
   padding: 0;
   height: 50px !important;
   line-height: 50px;
+  z-index: 1001;
 }
 
 .mobile-header-content {
@@ -401,6 +410,11 @@ onUnmounted(() => {
   padding: 0 10px;
 }
 
+.header-btn {
+  min-width: 44px;
+  min-height: 44px;
+}
+
 .mobile-title {
   font-size: 16px;
   font-weight: bold;