Background goal

Echo is a large app written in OOUI. We should port it to Codex, which should have most of the required components at this point. To the extent it doesn't, we should implement those components in Echo, and upstream them to Codex afterwards.

Echo also has complex state management, and would benefit from using Pinia(*).

(*) Pinia is now available in MediaWiki, see T326174.

Design proposal

These are the design proposals we worked during the Hackathon 2023:

	Option 1: using the Codex components and keeping the same structure as the current Notifications page and popup in production.
	Option 2: using the Codex components and improving some aspects of the current page in production.