style(layout): 完善 Z-INDEX 层级管理，优化全局侧边栏样式

layoutConfig.js:
- 重构 Z_INDEX 常量，分层管理（页面内部 → 系统级）
- 添加详细注释说明各层级用途
- 新增 SIDEBAR、DROPDOWN、POPOVER 等层级定义

GlobalSidebar:
- 使用统一的 Z_INDEX.SIDEBAR 常量
- 优化背景色和边框样式
- 添加 h="100%" 确保高度填满

MainLayout:
- 简化注释

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

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

src/components/GlobalSidebar/index.js

@@ -29,6 +29,7 @@ import { useNavigate } from 'react-router-dom';
 import { useGlobalSidebar } from '@/contexts/GlobalSidebarContext';
 import { useAuth } from '@/contexts/AuthContext';
 import { getEventDetailUrl } from '@/utils/idEncoder';
+import { Z_INDEX } from '@/layouts/config/layoutConfig';
 import WatchSidebar from '@views/Profile/components/WatchSidebar';
 import { WatchlistPanel, FollowingEventsPanel } from '@views/Profile/components/WatchSidebar/components';
 import HotSectorsRanking from '@views/Profile/components/MarketDashboard/components/atoms/HotSectorsRanking';
@@ -267,12 +268,14 @@ const GlobalSidebar = () => {
   return (
     <Box
       w={isOpen ? '300px' : '72px'}
+      h="100%"
       flexShrink={0}
       transition="width 0.2s ease-in-out"
       display={{ base: 'none', md: 'block' }}
-      bg="rgba(26, 32, 44, 0.6)"
-      borderLeft="1px solid rgba(255, 255, 255, 0.05)"
+      bg="rgba(26, 32, 44, 0.98)"
+      borderLeft="1px solid rgba(255, 255, 255, 0.08)"
       position="relative"
+      zIndex={Z_INDEX.SIDEBAR}
     >
       {/* 加载状态 */}
       {loading && (
@@ -287,8 +290,9 @@ const GlobalSidebar = () => {
           {/* 标题栏 - 收起按钮 + 标题 */}
           <HStack
             px={3}
-            py={2}
-            borderBottom="1px solid rgba(255, 255, 255, 0.05)"
+            py={3}
+            bg="rgba(26, 32, 44, 1)"
+            borderBottom="1px solid rgba(255, 255, 255, 0.1)"
             flexShrink={0}
           >
             <IconButton