style(layout): 优化页脚样式，移除冗余装饰元素

- 调整页脚内边距，移除背景透明设置，使用卡片背景色并添加顶部边框
- 简化页脚内容容器样式，移除圆角、边框和毛玻璃效果
- 调整文字颜色为次要文本色，减少间距，优化视觉层次
- 更新分隔符颜色以匹配边框颜色变量

frontend/src/layouts/AppLayout.tsx

@@ -466,29 +466,24 @@ export default function AppLayout() {
         <Footer
           style={{
             flexShrink: 0,
-            padding: "0 24px 16px",
+            padding: "16px 24px",
-            background: "transparent"
+            background: "var(--app-bg-card)",
+            borderTop: "1px solid var(--app-border-color)"
           }}
         >
           <div
             style={{
-              minHeight: 44,
-              borderRadius: 8,
-              border: "1px solid var(--app-border-color)",
-              background: "var(--app-bg-card)",
               display: "flex",
               alignItems: "center",
               justifyContent: "center",
-              gap: 16,
+              gap: 12,
-              padding: "10px 16px",
+              color: "var(--app-text-secondary)",
-              color: "var(--app-text-main)",
               fontSize: 13,
-              textAlign: "center",
+              textAlign: "center"
-              backdropFilter: "blur(10px)"
             }}
           >
             {platformConfig?.icpInfo ? <span>{platformConfig.icpInfo}</span> : null}
-            {platformConfig?.icpInfo && platformConfig?.copyrightInfo ? <span aria-hidden="true" style={{ color: "#d0d5dd" }}>|</span> : null}
+            {platformConfig?.icpInfo && platformConfig?.copyrightInfo ? <span aria-hidden="true" style={{ color: "var(--app-border-color)" }}>|</span> : null}
             {platformConfig?.copyrightInfo ? <span>{platformConfig.copyrightInfo}</span> : null}
           </div>
         </Footer>