Introduce a sync status indicator mode per Project

[janvlug]

Describe the bug
I see the Offline Mode Is On indicator in the left sidebar:

This gives the impression that all Projects are offline. However, this is not the case.
Some Projects still update the remote calendars.

There is for each Project in the context menu that you get when you right click a menu option: Refresh. For some Projects I see a spinner appearing when clicking this option, for some Projects not. The spinner does not appear for Projects with repeating tasks and Projects created from Nextcloud Deck calendars.

My suggestion is to indicate per Project if there is an issue with syncing, instead of having one indicator for all projects which indicates Offline Mode is On, which is actually also not correct, because the system is not offline

Desktop (please complete the following information):

• OS or DE: Fedora 40, GNOME
• Version: 4.7

This issue is related to issue #1215, but I tried to make this issue more actionable, as #1215 seems a combination of issues to me. See also #1256.