👑[Enhancement] 关于账号登录的问题

之前的时候手机号和QQ都登陆过，后来是用steam登录的，然后也是用steam账户进行的捐赠，现在想要手机号和QQ全部绑定到这个捐赠账号上，提示已经绑定别的账号了。能不能做个账号合并呢。

The issue requests merging phone- and QQ-linked accounts into the Steam account associated with a donation, but it names no files, entry points, or tests. Start by locating account linking and donation identity handling, then determine the rules for resolving accounts that are already linked. Done means the requested identities can be merged without losing the donation account, with coverage for the existing-account conflict.

Written by the indexing model from the issue text.