Redesign confirmation of learning buttons

Make this like duolingo, where a background color changes to green or something

The issue names no file, test, or entry point, so first locate the learning confirmation buttons in the frontend and inspect their current styling and state handling. Define the confirmation states and verify that the button background visibly changes to green or an equivalent success color when appropriate, matching the intended Duolingo-like behavior.

Written by the indexing model from the issue text.