Parent task state should reflect sub-task completion (including partial state)

[KodeBarista]

Issue/Description:
Currently, parent tasks and sub-tasks behave independently. A parent task can be marked as completed even if some or all of its sub-tasks are still incomplete. This creates an inconsistent and confusing state.

A more intuitive approach is for the parent task to reflect the completion state of its sub-tasks. For example:

• If no sub-tasks are completed -> parent remains unchecked
• If some sub-tasks are completed -> parent shows an indeterminate (partial) state
• If all sub-tasks are completed -> parent is marked as completed
• If user checks parent as completed -> all sub-tasks are checked as completed

This aligns with common task management patterns and improves clarity.

Other Notes:
This would improve UX by aligning the task hierarchy with user expectations and reducing inconsistent states between parent tasks and their sub-tasks.

[edfloreshz]

It would be great to have a partial completion state in the checkbox, this is something that can be proposed to libcosmic.

For now, I think we can adhere to the behavior you propose.

[edfloreshz]

I actually reported this way back in 2022

pop-os/libcosmic#24

[KodeBarista]

I've upvoted the issue in libcosmic