Allow setting a password after creating an account via OAuth

### Your use case

#### What would you like to do?
#### Why would you like to do it?

I created an account (via the Android application, but this doesn't seem to be relevant) using Google OAuth. Now I would like to use a desktop app that's somewhat lighter weight than one based on Electron (specifically Nheko), but all of them seem to require a password instead of using OAuth.

#### How would you like to achieve it?

Currently, all ways I can find to set a password demand the original password, which doesn't exist when authenticated via OAuth. The "Old password" field cannot be left blank, and anything I enter into that field naturally fails to match the nonexistent password. It seems to me that it should be possible to leave the old password blank when it doesn't exist, in order to add a password to an account that doesn't have one. (Possibly this should check to see if the user is authenticated via OAuth, in which case it might disable the old password field entirely.)

### Have you considered any alternatives?

I don't see much of an alternative except possibly forcing users to create passwords when creating an account via OAuth, but at least some users may be using OAuth specifically to avoid that.

### Additional context

_No response_

No files, tests, or entry points are named. Start by locating the password-setting flow and how OAuth-created accounts are represented; done means an OAuth-created account can add a password without an old-password value.

Written by the indexing model from the issue text.