feat(auth): redirect users to original page after login

[thostetler]

Summary

• After successful login, users are now redirected back to the page they were viewing when they initiated the login flow
• Navbar login link now includes the current path as a next query parameter
• Security: only relative paths are allowed (blocks external URLs and protocol-relative URLs)

Test plan

• Navigate to an abstract page, click Login, complete login → should return to abstract page
• Navigate to search results, click Login, complete login → should return to search results
• Try to inject external URL in next param → should be blocked, falls back to reload
• Existing auth routing E2E tests still pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.8%. Comparing base (9678f94) to head (6857d85).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #758     +/-   ##
========================================
- Coverage    71.8%   71.8%   -0.0%     
========================================
  Files         217     217             
  Lines       24485   24485             
  Branches     1402    1402             
========================================
- Hits        17575   17572      -3     
- Misses       6878    6881      +3     
  Partials       32      32             

see 2 files with indirect coverage changes

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