Load the API user after sign-in so an admin is recognised at once

[pkvach]

Fixes #2205

Sign-in endpoints answer with the auth library's user, which keeps the admin flag under attrs. The widget stored that user as-is, so it treated an admin as a regular user until the next page load. The edit countdown on an admin's freshly posted comment came from that.

• Anonymous, email and Telegram sign-in now fetch /user once the endpoint succeeds and return that user. The fetch skips the /auth/status probe that getUser makes first, because that probe tells nothing new right after a successful sign-in.
• OAuth is unchanged: it already loads the user through getUser after the popup closes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.23%. Comparing base (d9254d2) to head (74f232a).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2260      +/-   ##
==========================================
+ Coverage   74.01%   74.23%   +0.21%     
==========================================
  Files         129      129              
  Lines        3745     3749       +4     
  Branches      830      829       -1     
==========================================
+ Hits         2772     2783      +11     
+ Misses        967      960       -7     
  Partials        6        6              

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