fix: consistently treat returnTo parameter as an absolute path

[guabu]

📋 Changes

Currently the returnTo parameter is inconsistently treated: sometimes it's a URL (e.g.: https://example.com/dashboard) and other times as a path (/dashboard). This results in subtle bugs when building on the assumption that it is always an absolute path.

This PR ensure that all instances of the returnTo parameter are treated as absolute paths.

This was discovered as a bug with the basePath handling where the returnTo parameter was assumed to be an absolute path (e.g.: /base-path/dashboard). In reality, the host was prepended when constructing the returnTo parameter resulting in: http://localhost:3000/base-path/dashboard. With the basePath enabled, this generated malformed URLs such as /base-path/http://localhost:3000/base-path/dashboard. This can only be reproduced when a base path is set.

🎯 Testing

1. Add a base path to your application
2. Specify a returnTo path in your login URL
3. Notice the resulting redirect is malformed

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.91%. Comparing base (3d0f19e) to head (14ff06b).
Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2185      +/-   ##
==========================================
+ Coverage   82.88%   82.91%   +0.02%     
==========================================
  Files          21       21              
  Lines        2092     2095       +3     
  Branches      372      372              
==========================================
+ Hits         1734     1737       +3     
  Misses        351      351              
  Partials        7        7              

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