feat(theme): 移除首页主题切换按钮，恢复默认深色主题 ClassicDark

- 首页 index.vue：删除主题切换按钮 DOM、useTheme 引入及 handleToggleTheme 处理函数
- nuxt.config.ts：defaultTheme ClassicLight → ClassicDark；enabledThemes 恢复为 4 主题（System/ClassicDark/ClassicLight/ModernLight）；x-default-theme meta 和防闪烁脚本回退值同步改为 ClassicDark
- schema.ts / system-settings-defaults / site-config API / POST API / useSiteConfig：7 处主题默认配置全部从 ClassicLight/双浅色列表 同步改为 ClassicDark/四主题完整列表

app/pages/index.vue

@@ -33,18 +33,6 @@
         <!-- 用户信息区域 -->
         <div class="user-section">
           <ClientOnly>
-            <!-- 主题切换按钮 -->
-            <button
-              class="utility-btn theme-toggle-btn"
-              type="button"
-              aria-label="主题"
-              title="切换主题"
-              @click="handleToggleTheme"
-            >
-              <Icon :name="isDark ? 'sun' : 'moon'" :size="18" />
-              <span class="utility-btn-label">主题</span>
-            </button>
-
             <div v-if="isClientAuthenticated" class="user-info">
               <div class="user-details-desktop">
                 <span class="user-name">{{ user?.name || '用户' }}</span>
@@ -686,19 +674,12 @@ import ConfirmDialog from '~/components/UI/ConfirmDialog.vue'
 
 import { useNotifications } from '~/composables/useNotifications'
 import { useSiteConfig } from '~/composables/useSiteConfig'
-import { useTheme } from '~/composables/useTheme'
 import CustomSelect from '~/components/UI/Common/CustomSelect.vue'
 
 // 获取运行时配置
 const config = useRuntimeConfig()
 const router = useRouter()
 
-// 主题
-const { isDark, toggleTheme } = useTheme()
-const handleToggleTheme = () => {
-  toggleTheme()
-}
-
 // 站点配置
 const {
   siteTitle,