fix: OPTIC-1472: Next parameter isn't being set on /user/login after failed auth

[mcanu]

PR fulfills these requirements

• Commit message(s) and PR title follows the format [fix|feat|ci|chore|doc]: TICKET-ID: Short description of change made ex. fix: DEV-XXXX: Removed inconsistent code usage causing intermittent errors
• Tests for the changes have been added/updated (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)
• Best efforts were made to ensure docs/code are concise and coherent (checked for spelling/grammatical errors, commented out code, debug logs etc.)
• Self-reviewed and ran all changes on a local instance (for bug fixes/features)

Change has impacts in these area(s)

(check all that apply)

• Product design
• Backend (Database)
• Backend (API)
• Frontend

Describe the reason for change

Users weren't redirected properly to /user/login when logged out and trying to access an authed page.
This caused the next= parameter to be lost, losing proper redirection after login/signup.

What does this fix?

Unauthed users trying to visit an authed page get properly redirected after login

What is the current behavior?

Users are always redirected to projects page.

Does this PR introduce a breaking change?

(check only one)

• Yes, and covered entirely by feature flag(s)
• Yes, and covered partially by feature flag(s)
• No
• Not sure (briefly explain the situation below)

What level of testing was included in the change?

(check all that apply)

• e2e
• integration
• unit

Which logical domain(s) does this change affect?

Authentication

signup.lso.mov

login.lso.mov

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.78%. Comparing base (ea7294a) to head (077378a).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
label_studio/users/views.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #6813   +/-   ##
========================================
  Coverage    76.78%   76.78%           
========================================
  Files          171      171           
  Lines        14023    14024    +1     
========================================
+ Hits         10767    10768    +1     
  Misses        3256     3256           

Flag	Coverage Δ
pytests	76.78% <75.00%> (+<0.01%)	⬆️

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