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/App.vue

@@ -56,7 +56,7 @@ html, body {
   width: 100%;
   margin: 0;
   padding: 0;
-  overflow-x: hidden;
+  overflow: hidden; /* 完全隐藏滚动条 */
 }
 
 body {
@@ -83,7 +83,7 @@ body {
   width: 100vw;
   height: 100vh;
   min-height: 100vh;
-  z-index: -2;
+  z-index: -10; /* 确保在最底层 */
   pointer-events: none;
   background-attachment: fixed;
 }
@@ -106,16 +106,13 @@ main.with-navbar {
   min-height: 100vh;
 }
 
-/* 登录页面特殊背景处理 - 参考Welcome页面样式 */
+/* 登录页面特殊背景处理 */
 #app[data-route="Login"] .fullscreen-background {
   background: url('/images/backgrounds/login.png') center/cover no-repeat;
   background-attachment: fixed;
-  min-height: 100vh;
-  position: fixed;
-  top: 0;
-  left: 0;
-  width: 100vw;
-  height: 100vh;
+  background-size: cover;
+  background-position: center;
+  background-repeat: no-repeat;
 }
 
 /* ========== 页面特殊样式 ========== */

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 {