feat(config): 更新系统配置默认值和文档说明

- 将 allowCustomChannel 默认值从 false 改为 true
- 修改 allowCustomChannel 字段类型为指针类型以支持默认值设置
- 更新相关文档中对 allowCustomChannel 功能的描述
- 修复画布页面中对话框节点打开逻辑，避免文本节点意外触发编辑对话框
- 将用户配置中的渠道模式默认值从 remote 改为 local

service/settings.go

@@ -33,6 +33,10 @@ func normalizePublicSetting(setting model.PublicSetting) model.PublicSetting {
 	if setting.ModelChannel.AvailableModels == nil {
 		setting.ModelChannel.AvailableModels = []string{}
 	}
+	if setting.ModelChannel.AllowCustomChannel == nil {
+		enabled := true
+		setting.ModelChannel.AllowCustomChannel = &enabled
+	}
 	return setting
 }