Change LoginHandler validation error from http.Error (plain text, mismatched Content-Type) to mw.RespondError (proper JSON {error:...} response). Add TestLogin_EmptyFields and TestLogin_MissingEmail tests verifying JSON error format.
Change LoginHandler validation error from http.Error (plain text, mismatched Content-Type) to mw.RespondError (proper JSON {error:...} response). Add TestLogin_EmptyFields and TestLogin_MissingEmail tests verifying JSON error format.