Multiple accounts with the same localpart conflicts with browser autofill

I have multiple Matrix accounts all with the same `madlittlemods` localpart:

- `@madlittlemods:matrix.org`
- `@madlittlemods:gitter.im`
- etc

The problem is that if I want to sign in from both of those accounts in Hydrogen since it supports multiple accounts, Chrome's autofill will ask to update the password for the other account since it considers `madlittlemods` as the username for both.


## Potential solutions

Sign in with full MXID. Related issues:

- https://github.com/vector-im/element-android/issues/5615
- https://github.com/vector-im/element-web/issues/6826

Start by reproducing the issue through Hydrogen's sign-in flow with two accounts sharing a localpart, then trace how the form exposes the username to browser autofill. Confirm the change using Chrome with both accounts; done means each account is stored distinctly and updating one password no longer targets the other.

Written by the indexing model from the issue text.