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

nuxt.config.ts

@@ -156,8 +156,8 @@ export default defineNuxtConfig({
         themeColor: process.env.PWA_THEME_COLOR || '#111111'
       },
       // 主题配置
-      defaultTheme: process.env.NUXT_PUBLIC_DEFAULT_THEME || 'ClassicLight',
-      enabledThemes: (process.env.NUXT_PUBLIC_ENABLED_THEMES || 'ClassicLight,ModernLight')
+      defaultTheme: process.env.NUXT_PUBLIC_DEFAULT_THEME || 'ClassicDark',
+      enabledThemes: (process.env.NUXT_PUBLIC_ENABLED_THEMES || 'System,ClassicDark,ClassicLight,ModernLight')
         .split(',')
         .map((s) => s.trim())
         .filter(Boolean),
@@ -192,7 +192,7 @@ export default defineNuxtConfig({
         { name: 'theme-color', content: '#111111' },
         {
           name: 'x-default-theme',
-          content: process.env.NUXT_PUBLIC_DEFAULT_THEME || 'ClassicLight'
+          content: process.env.NUXT_PUBLIC_DEFAULT_THEME || 'ClassicDark'
         },
         { name: 'apple-mobile-web-app-capable', content: 'yes' },
         { name: 'apple-mobile-web-app-status-bar-style', content: 'black-translucent' },
@@ -213,7 +213,7 @@ export default defineNuxtConfig({
             `      var meta=document.querySelector('meta[name="x-default-theme"]');`,
             `      if(meta && meta.content){s=meta.content}`,
             `    }catch(e){s=null}`,
-            `    if(!s){s='ClassicLight'}`,
+            `    if(!s){s='ClassicDark'}`,
             `  }`,
             `  if(s==='System'||s==='system'){eff=window.matchMedia('(prefers-color-scheme: dark)').matches?'dark':'light'}`,
             `  else if(typeof s==='string'){eff=s.endsWith('Dark')||s==='dark'?'dark':(s.endsWith('Light')||s==='light'?'light':s)}`,