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/utils/system-settings-defaults.ts

@@ -68,6 +68,6 @@ export const SYSTEM_SETTINGS_DEFAULTS = {
   autoBackupRetentionDays: 30,
   statisticsCode: '',
   statisticsCodeEnabled: false,
-  defaultTheme: 'ClassicLight',
-  enabledThemes: '["ClassicLight","ModernLight"]'
+  defaultTheme: 'ClassicDark',
+  enabledThemes: '["System","ClassicDark","ClassicLight","ModernLight"]'
 } as const