feat: CalDAV section support via VTODO List prefix

[alainm23]

Adds section support for CalDAV projects.

Sections are represented as VTODOs with the List : prefix in the summary, following the same convention used by Nextcloud Deck. Tasks inside a section use RELATED-TO pointing to the section's UID, making it compatible with other
CalDAV clients.

What's new:

• Create, rename and delete sections in CalDAV projects
• Sections sync bidirectionally — changes made in other clients (Nextcloud Tasks, Thunderbird, etc.) are reflected in Planify after sync
• Tasks moved between sections sync correctly in both directions
• Deleting a section also deletes all its tasks on the server
• Tasks in a section appear correctly nested in other CalDAV clients

Fixes: #1356 #1700 #1355

[github-actions]

Translation files need updating

• Apply translation updates