fix: 修复一级导航菜单展开时文字颜色消失问题

- 添加 getMenuButtonStyle 函数统一管理 MenuButton 样式
- 为所有 MenuButton 添加 _expanded 状态样式
- 菜单展开时保持浅色主题（背景 gray.100，文字 gray.700）

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

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

src/components/Navbars/components/Navigation/DesktopNav.js

@@ -44,6 +44,12 @@ const DesktopNav = memo(({ isAuthenticated, user }) => {
         _focus: { bg: 'gray.100', color: 'gray.900' },
     };
 
+    // MenuButton 通用样式（菜单展开时保持浅色主题）
+    const getMenuButtonStyle = (isActiveState) => ({
+        _hover: { bg: isActiveState ? 'blue.700' : 'gray.100' },
+        _expanded: { bg: isActiveState ? 'blue.700' : 'gray.100', color: isActiveState ? 'white' : menuTextColor },
+    });
+
     // 🎯 初始化导航埋点Hook
     const navEvents = useNavigationEvents({ component: 'top_nav' });
 
@@ -74,7 +80,7 @@ const DesktopNav = memo(({ isAuthenticated, user }) => {
                     border={isActive(['/community', '/concepts']) ? '2px solid' : 'none'}
                     borderColor={isActive(['/community', '/concepts']) ? 'blue.300' : 'transparent'}
                     borderRadius="md"
-                    _hover={{ bg: isActive(['/community', '/concepts']) ? 'blue.700' : 'gray.50' }}
+                    {...getMenuButtonStyle(isActive(['/community', '/concepts']))}
                     onMouseEnter={highFreqMenu.handleMouseEnter}
                     onMouseLeave={highFreqMenu.handleMouseLeave}
                     onClick={highFreqMenu.handleClick}
@@ -146,7 +152,7 @@ const DesktopNav = memo(({ isAuthenticated, user }) => {
                     border={isActive(['/limit-analyse', '/stocks', '/trading-simulation']) ? '2px solid' : 'none'}
                     borderColor={isActive(['/limit-analyse', '/stocks', '/trading-simulation']) ? 'blue.300' : 'transparent'}
                     borderRadius="md"
-                    _hover={{ bg: isActive(['/limit-analyse', '/stocks', '/trading-simulation']) ? 'blue.700' : 'gray.50' }}
+                    {...getMenuButtonStyle(isActive(['/limit-analyse', '/stocks', '/trading-simulation']))}
                     onMouseEnter={marketReviewMenu.handleMouseEnter}
                     onMouseLeave={marketReviewMenu.handleMouseLeave}
                     onClick={marketReviewMenu.handleClick}
@@ -224,7 +230,7 @@ const DesktopNav = memo(({ isAuthenticated, user }) => {
                     border={isActive(['/agent-chat', '/value-forum']) ? '2px solid' : 'none'}
                     borderColor={isActive(['/agent-chat', '/value-forum']) ? 'blue.300' : 'transparent'}
                     borderRadius="md"
-                    _hover={{ bg: isActive(['/agent-chat', '/value-forum']) ? 'blue.700' : 'gray.50' }}
+                    {...getMenuButtonStyle(isActive(['/agent-chat', '/value-forum']))}
                     onMouseEnter={agentCommunityMenu.handleMouseEnter}
                     onMouseLeave={agentCommunityMenu.handleMouseLeave}
                     onClick={agentCommunityMenu.handleClick}
@@ -300,7 +306,7 @@ const DesktopNav = memo(({ isAuthenticated, user }) => {
                     variant="ghost"
                     color={menuTextColor}
                     rightIcon={<ChevronDown size={16} />}
-                    _hover={{ bg: 'gray.50' }}
+                    {...getMenuButtonStyle(false)}
                     onMouseEnter={contactUsMenu.handleMouseEnter}
                     onMouseLeave={contactUsMenu.handleMouseLeave}
                     onClick={contactUsMenu.handleClick}