[SystemKeeper]

Trying again, lost the original one to githubs 500...

Before we did uploading and sharing completely synchronously. To have a better UX, we

• Keep the probing synchronously
• Put the uploading of the files on a background URLSession (so it is done without extension or app running)
• The background URLSession still calls delegates, either in the ShareExtension, in case it is still running, or on the main app (so we need to handle both cases)
• Only on the last file delegate (in case multiple are shared) we share the files to the conversation. This keeps the ordering and caption. It would be better with Add bulk attachment sharing endpoint spreed#19946 of course.
• If something fails in the background, we show a local notification.
• Opening the local notification opens the conversation and shows the share dialog to retry sharing.
• Having multiple failed attempts from different shares (so different conversation and/or different upload group) is something we might expand on later, but that is something I would not expect to happen too often

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI (N/A)