refactor(layout): 统一 z-index 配置

- layoutConfig.js: 添加 Z_INDEX.FOOTER (1000，与 NAVBAR 同级)
- HomeNavbar.js: zIndex 从硬编码 1000 改为 Z_INDEX.NAVBAR
- AppFooter.js: 添加 zIndex={Z_INDEX.FOOTER}

导航栏和页脚的 z-index 现在统一在 layoutConfig.js 中配置。

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/layouts/AppFooter.js

@@ -1,7 +1,7 @@
 import React from 'react';
 import { Box, VStack, HStack, Text, Link, useColorModeValue } from '@chakra-ui/react';
 import RiskDisclaimer from '../components/RiskDisclaimer';
-import { LAYOUT_PADDING } from './config/layoutConfig';
+import { LAYOUT_PADDING, Z_INDEX } from './config/layoutConfig';
 
 /**
  * 应用通用页脚组件
@@ -9,7 +9,7 @@ import { LAYOUT_PADDING } from './config/layoutConfig';
  */
 const AppFooter = () => {
   return (
-    <Box bg={useColorModeValue('gray.100', 'gray.800')} py={2} px={LAYOUT_PADDING.x}>
+    <Box bg={useColorModeValue('gray.100', 'gray.800')} py={2} px={LAYOUT_PADDING.x} zIndex={Z_INDEX.FOOTER} position="relative">
         <VStack spacing={1}>
           <RiskDisclaimer />
           <Text color="gray.500" fontSize="sm">