phsl / new-api

* backend
  - model: add `Remark` field (varchar 255, `json:"remark,omitempty"`); AutoMigrate handles schema change automatically
  - controller:
    * accept `remark` on user create/update endpoints
    * hide remark from regular users (`GetSelf`) by zero-ing the field before JSON marshalling
    * clarify inline comment explaining the omitempty behaviour

* frontend (React / Semi UI)
  - AddUser.js & EditUser.js: add “Remark” input for admins
  - UsersTable.js:
    * remove standalone “Remark” column
    * show remark as a truncated Tag next to username with Tooltip for full text
    * import Tooltip component
  - i18n: reuse existing translations where applicable

This commit enables administrators to label users with private notes while ensuring those notes are never exposed to the users themselves.