refactor(settings): 规范化缩写词命名并优化前端帮助界面

- 后端：将 Smtp/Api/Doc 字段改为 SMTP/API/Doc（遵循 Go 命名规范）
- 前端：添加 Gemini 帮助按钮，简化配额说明展示

backend/internal/handler/setting_handler.go

@@ -39,9 +39,9 @@ func (h *SettingHandler) GetPublicSettings(c *gin.Context) {
 		SiteName:            settings.SiteName,
 		SiteLogo:            settings.SiteLogo,
 		SiteSubtitle:        settings.SiteSubtitle,
-		ApiBaseUrl:          settings.ApiBaseUrl,
+		APIBaseURL:          settings.APIBaseURL,
 		ContactInfo:         settings.ContactInfo,
-		DocUrl:              settings.DocUrl,
+		DocURL:              settings.DocURL,
 		Version:             h.version,
 	})
 }