✅ Add share login handling tests

store.go

@@ -166,6 +166,7 @@ func (u User) merge(updates User) (User, error) {
 }
 
 var ErrShareNotFound = errors.New("share not found")
+var ErrLoginDuplicate = errors.New("login already exists")
 
 func (store *DBStore) AddUser(user User) (err error) {
 	if strings.Contains(user.Username, ":") {