Adjust login to be friendlier to saving and restoring passwords

Currently, the highly dynamic nature of login makes it hard for clients other than web browsers that see "that's a username and that's a password field, let's autocomplete" to fill passwords, or better yet, save login details in the OS-provided wallet and attempt to create a new session when the old one expires

The issue names no files, tests, or entry points. Start by locating the protocol's login and session schemas, then determine the scope of changes needed for password-manager compatibility and session restoration; done should be defined by an agreed protocol design and corresponding validation.

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