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

server/api/site-config.get.ts

@@ -90,8 +90,8 @@ export default defineEventHandler(async (event) => {
           forceBlockAllRequests: false,
           enableReplayRequests: false,
           smtpEnabled: false,
-          defaultTheme: 'ClassicLight',
-          enabledThemes: '["ClassicLight","ModernLight"]',
+          defaultTheme: 'ClassicDark',
+          enabledThemes: '["System","ClassicDark","ClassicLight","ModernLight"]',
           scheduleDaysBeforeEnabled: false,
           scheduleDaysBefore: 30,
           scheduleDaysAfterEnabled: false,