Don't lose annotations without warning

[jospoortvliet]

The new annotations feature has a downside: when you annotate a document and then close it, your changes aren't saved, nor is there a warning. This has lost me quite some work a few times already 😟

It'd be good to auto-save on close, or at least warn.

Steps to reproduce

1. Annotate a PDF
2. Close by the X
3. Open it

Expected behaviour

Annotations should be there

Actual behaviour

They are gone

Firefox, latest Nextcloud hub 28.0.3.1 on cloud.nextcloud.com ;-)

[szaimen]

Cc @danxuliu