fix: prioritize registration errors over inline validations

[arbrandes]

Description

• Registration errors from form submission (e.g., "password too similar to username") were being masked by stale inline validation results
• The backendValidations memo in RegisterContext checked state.validations (set during on-blur inline validation) before state.registrationError (set on form submission failure), so submission errors were never surfaced to the user
• Swapped the priority so registration errors take precedence, and added a unit test covering the scenario

How to reproduce

1. Go to the registration page
2. Fill out all fields, using a password that contains the username (e.g., username testuser, password testuser123)
3. Submit the form
4. The server returns a "The password is too similar to the username." error, but no error message is displayed next to the password field

How to test

1. Go to the registration page
2. Fill out all fields, using a password that contains the username (e.g., username testuser, password testuser123)
3. Submit the form
4. The error message "The password is too similar to the username." is now displayed next to the password field

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.02%. Comparing base (642853e) to head (eb1747e).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1640      +/-   ##
==========================================
+ Coverage   87.58%   91.02%   +3.43%     
==========================================
  Files         125      110      -15     
  Lines        2312     2350      +38     
  Branches      649      666      +17     
==========================================
+ Hits         2025     2139     +114     
+ Misses        278      204      -74     
+ Partials        9        7       -2     

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[jesusbalderramawgu]

LGTM!