refactor(ui): migrate auth flows from ConnectRPC to REST API

[rpelczar]

fixes #6281

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.96%. Comparing base (c3115d4) to head (d9bc4f9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6292   +/-   ##
=======================================
  Coverage   57.96%   57.96%           
=======================================
  Files         496      496           
  Lines       41602    41602           
=======================================
  Hits        24115    24115           
  Misses      16033    16033           
  Partials     1454     1454           

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