CI Shield setup: can't connect to database

Check your MySQLi my.ini port number it is usually 3306

Thanks for the suggestion, but it didn't help: for whatever reason, MAMP sets its default ports for Apache and MySQL at 8888 and 8889 respectively, and I have not changed those settings. I was also able to access the database through localhost:8889 through other scripts and the terminal, so that doesn't appear to be the problem.

Are you using MAMP for Windows? Even the free version of MAMP for Mac should allow you to use ports 80 and 3306 (web server & Db respectively).
I had published an end-to-end tutorial video on showing how to set up the free version of MAMP (on macOS) for frameworks like CodeIgniter:

How to Set Up a Virtual Host on Free MAMP for Frameworks like CodeIgniter 4 and Laravel  

Near the exact timestamp suffixed to the link above, you should see where exactly to customize default MAMP ports.

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CodeIgniter Wizard (CRUD code generator for Mac) instantly scaffolds Bootstrap-based web applications with an administrative interface (admin templates include Bootstrap5)

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