修复登录页面布局和全屏覆盖 - 恢复容器结构，优化背景层级，消除白边

demo/frontend/src/views/Login.vue

@@ -1,9 +1,10 @@
 <template>
-  <!-- Logo -->
-  <div class="logo">Logo</div>
-  
-  <!-- 登录卡片 -->
-  <div class="login-card">
+  <div class="login-page">
+    <!-- Logo -->
+    <div class="logo">Logo</div>
+    
+    <!-- 登录卡片 -->
+    <div class="login-card">
       <!-- Logo图标 -->
       <div class="card-logo">
         <div class="logo-icon">Logo</div>
@@ -83,6 +84,7 @@
         </div>
       </div>
     </div>
+  </div>
 </template>
 
 <script setup>
@@ -201,7 +203,20 @@ const handleLogin = async () => {
 </script>
 
 <style scoped>
-/* 登录页面样式 - 背景由App.vue处理 */
+.login-page {
+  min-height: 100vh;
+  width: 100vw;
+  height: 100vh;
+  background: transparent;
+  position: fixed;
+  top: 0;
+  left: 0;
+  overflow: hidden;
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
+  margin: 0;
+  padding: 0;
+  z-index: 1;
+}
 
 /* 左上角Logo */
 .logo {