Tidy up _AppsDrawer.pcss and conform it to our naming policy

[luixxiul]

Your use case

What would you like to do?

Tidy up _AppsDrawer.pcss and conform it to our naming policy.

Why would you like to do it?

• To increase readability and maintainability, making it easy to update the style which is not consistent with other parts of UI and almost out of date. Naming inconsistency has discouraged us to update the widget card design since several years.
• To conduct Extract style rules specific to AppTile from _AppsDrawer.pcss #25269
• To avoid a regression such as Make widgets take up the full height of the PiP again matrix-org/matrix-react-sdk#10886, we need our naming policy's strict application

How would you like to achieve it?

For the sake of reviewers, with several commits and PRs.

Have you considered any alternatives?

No response

Additional context

No response