Show a small notification banner/toast when not checked for outdated packages

With the incoming ability to prevent automatic checks for outdated packages, there would be nice to have a small message that mentions this to the user when they haven't manually checked for updates yet.

This notification could potentially be implemented with a small banner somewhere in the application, or with just a small message at the bottom.

No files, tests, or entry points are named. Start by locating the existing outdated-package check and the setting that prevents automatic checks, then identify the application's notification UI pattern. Done means users who have not manually checked for updates see a small banner or bottom message, with the behavior covered by the relevant checks.

Written by the indexing model from the issue text.