Make sure CORS works when client is deployed to a subdir

[infomiho]

Describe the bug
We support client.baseDir option as a way for users to deploy the client app in a sub-directory. Users need to set the WASP_WEB_CLIENT_URL env variable on the server to an URL that includes the sub-directory i.e. https://<something>/subdir. Wasp then uses this full URL in the CORS settings. This breaks Wasp's CORS setup since subdirs are not considered different origins and this setup is wrong. Wasp should in this case set the allowed origin to just https://<something> (without the subdir part).

To Reproduce
Steps to reproduce the behavior:

1. Use the client.baseDir option
2. Deploy the client to a sub-directory e.g. Github Pages
3. Try to access the server from the client

Expected behavior
CORS should work.

[Prince1895]

Hi! I’d like to work on this issue if it’s available.
From what I understand, the fix involves adjusting how WASP_WEB_CLIENT_URL is used for CORS.
Please let me know if I can take this one.