feat(dark-mode): 实现深色模式支持

- 定义87个CSS变量的深色值（[data-theme=dark]选择器）
- 升级ThemeProvider支持light/dark/system三种模式
- 新增ThemeToggle组件（桌面端Header+移动端菜单）
- 添加防FOUC内联脚本（渲染前应用主题）
- Logo根据主题自动切换（logo.svg/logo-white.svg）
- 更新测试用例覆盖主题切换逻辑

src/components/layout/header.tsx

@@ -7,6 +7,8 @@ import { usePathname } from 'next/navigation';
 import { Menu, X } from 'lucide-react';
 import { AnimatePresence, motion } from 'framer-motion';
 import { Button } from '@/components/ui/button';
+import { ThemeToggle } from '@/components/ui/theme-toggle';
+import { useTheme } from '@/contexts/theme-context';
 import { COMPANY_INFO, NAVIGATION_V2, MEGA_DROPDOWN_DATA, type NavigationItemV2 } from '@/lib/constants';
 import { MegaDropdown } from '@/components/layout/mega-dropdown';
 import { useFocusTrap } from '@/hooks/use-focus-trap';
@@ -17,6 +19,7 @@ function HeaderContent() {
   const [openDropdown, setOpenDropdown] = useState<string | null>(null);
   const pathname = usePathname();
   const focusTrapRef = useFocusTrap<HTMLDivElement>(isOpen);
+  const { resolvedTheme } = useTheme();
 
   useEffect(() => {
     const handleScroll = () => {
@@ -90,7 +93,7 @@ function HeaderContent() {
               aria-label="返回首页"
             >
               <Image 
-                src="/logo.svg" 
+                src={resolvedTheme === 'dark' ? '/logo-white.svg' : '/logo.svg'}
                 alt={COMPANY_INFO.name}
                 width={120}
                 height={30}
@@ -146,6 +149,7 @@ function HeaderContent() {
             </nav>
 
             <div className="hidden md:flex items-center gap-3">
+              <ThemeToggle />
               <Button 
                 size="sm"
                 asChild
@@ -222,6 +226,10 @@ function HeaderContent() {
                   </motion.div>
                 ))}
                 <div className="mt-6 px-4 pt-6 border-t border-[var(--color-border-primary)]">
+                  <div className="flex items-center justify-between mb-4">
+                    <span className="text-sm text-[var(--color-text-muted)]">外观模式</span>
+                    <ThemeToggle />
+                  </div>
                   <Button
                     className="w-full"
                     asChild