feat: invitation accept screen and api

internal/domain/user/userDomain.go

@@ -14,6 +14,7 @@ var ErrAlreadyVerified = errors.New("user already verified")
 var ErrInvalidPasswordResetCode = errors.New("invalid password reset code")
 var ErrPasswordResetCodeExpired = errors.New("password reset code expired")
 var ErrInvalidDisplayName = errors.New("invalid display name")
+var ErrUsernameTaken = errors.New("username taken")
 
 type User struct {
 	ID                      uint32    `json:"id"`