User management improvements (#1101)

* Show more descriptive success messages for User actions

* Check username uniqueness when creating/updating User

* Adjust translations

* Add tests for `validateUsernameUnique()`

Co-authored-by: Deluan <deluan@navidrome.org>

persistence/user_repository.go

@@ -134,6 +134,9 @@ func (r *userRepository) Save(entity interface{}) (string, error) {
 		return "", rest.ErrPermissionDenied
 	}
 	u := entity.(*model.User)
+	if err := validateUsernameUnique(r, u); err != nil {
+		return "", err
+	}
 	err := r.Put(u)
 	if err != nil {
 		return "", err
@@ -157,6 +160,9 @@ func (r *userRepository) Update(entity interface{}, cols ...string) error {
 	if err := validatePasswordChange(u, usr); err != nil {
 		return err
 	}
+	if err := validateUsernameUnique(r, u); err != nil {
+		return err
+	}
 	err := r.Put(u)
 	if err == model.ErrNotFound {
 		return rest.ErrNotFound
@@ -186,6 +192,20 @@ func validatePasswordChange(newUser *model.User, logged *model.User) error {
 	return nil
 }
 
+func validateUsernameUnique(r model.UserRepository, u *model.User) error {
+	usr, err := r.FindByUsername(u.UserName)
+	if err == model.ErrNotFound {
+		return nil
+	}
+	if err != nil {
+		return err
+	}
+	if usr.ID != u.ID {
+		return &rest.ValidationError{Errors: map[string]string{"userName": "ra.validation.unique"}}
+	}
+	return nil
+}
+
 func (r *userRepository) Delete(id string) error {
 	usr := loggedUser(r.ctx)
 	if !usr.IsAdmin {