feat(settings): add home content customization and config injection

- Add home_content setting for custom homepage (HTML or iframe URL)
- Inject public settings into index.html to eliminate page flash
- Support ETag caching with automatic invalidation on settings update
- Add Vite plugin for dev mode settings injection
- Refactor HomeView to use appStore instead of local API calls

backend/internal/service/domain_constants.go

@@ -84,6 +84,7 @@ const (
 	SettingKeyAPIBaseURL   = "api_base_url"  // API端点地址（用于客户端配置和导入）
 	SettingKeyContactInfo  = "contact_info"  // 客服联系方式
 	SettingKeyDocURL       = "doc_url"       // 文档链接
+	SettingKeyHomeContent  = "home_content"  // 首页内容（支持 Markdown/HTML，或 URL 作为 iframe src）
 
 	// 默认配置
 	SettingKeyDefaultConcurrency = "default_concurrency" // 新用户默认并发量