Allow adding account via full username

### Your use case

#### What would you like to do?
Allow users to enter their full username (user:domain.com) rather than specifying to edit the home server, type in a server, and then need to enter the username.

#### Why would you like to do it?
Reduce less technical user confusion and number of steps to make the client more accessible.

#### How would you like to achieve it?
There could be a radio button for custom home server and a text box beside it (that you could click on, and have the radio button change automatically) where you type user:domain.com or user@domain.com and then you click connect, and you enter your password. Far less steps this way, less confusion.

### Have you considered any alternatives?

I know you can edit the config file to change the default home server, but this is even harder for family/friends to do.

### Additional context

_No response_

Start by reviewing the client account-add flow and the existing home-server configuration path; no source file or test is identified in the issue. Define how full usernames such as user:domain.com and user@domain.com should be handled, including the custom home-server choice. Done means a user can enter the full username, connect, and proceed to password entry with fewer steps.

Written by the indexing model from the issue text.