feat(user): require email when admin creates a user

- Add `email` as a required field in `createUserRequest`
- Enforce email format verification and database uniqueness checks in the admin user creation handler
- Update the admin user creation frontend modal with validation and form field
- Update the corresponding backend unit tests and regenerate Swagger docs

internal/apps/admin/user/errs.go

@@ -16,4 +16,6 @@ const (
 	usernameRequired = "用户名不能为空"
 	passwordTooShort = "密码长度不能少于 8 位" //nolint:gosec // false positive: this is an error message, not hardcoded credentials
 	createUserFailed = "创建用户失败"
+	emailRequired    = "邮箱不能为空"
+	emailExists      = "邮箱已被注册"
 )