Display a local notification if a message failed to be sent in background

We did it when doing a quick reply to a notification or when we post an image from Photos.app.
We should generalise it when a message was posted within the app and the app has been put in background.

A useful case is when posting a message to an e2e room. The sending can fail because some new devices require validation from the user.

The issue names no files or tests. Start by locating the existing local-notification handling for quick replies and Photos.app posts, then trace how an in-app message send reports failure after the app enters the background. Done means failed background sends produce a local notification, including failures caused by pending device validation in an e2e room.

Written by the indexing model from the issue text.