User-Agent reverts to "Anonymous" for token-authenticated clients constructed without a username

[duckduckgrayduck]

How to replicate:

from squarelet import SquareletClient

good = SquareletClient(base_uri=BASE_URI, username=USER, password=PW)

# good UA -> "... <username>"   (correct)

rebuilt = SquareletClient(base_uri=BASE_URI)   # no creds -> "... Anonymous"
rebuilt.refresh_token = good.refresh_token
rebuilt._set_tokens()

print(rebuilt.session.headers["User-Agent"])       # "... Anonymous"  <-- bug
print(rebuilt.session.headers["Authorization"])    # valid Bearer token

Proposed fix

Rebuild the UA deterministically from a captured base on every auth-state change, and when there's no local username, resolve one from users/me/ (cached). This labels any authenticated client correctly regardless of how it acquired its token.