Enhancement: Option to hide completed tasks in the navigation sidebar

Is your feature request related to a problem? Please describe.

When tasks are marked as completed, they remain permanently visible in the left navigation sidebar (under the respective calendar/list). Over time, this leads to a cluttered sidebar where completed tasks mix with open ones, making it harder to get a quick overview of what still needs to be done.

Describe the solution you'd like

An option to hide completed tasks from the left navigation sidebar — similar to how the Deck app handles archived cards. Ideally this would be a toggle (show/hide completed tasks) either per list or as a global setting, so users can choose whether completed tasks should be visible in the navigation or not.

Describe alternatives you've considered

• Moving completed tasks to a separate calendar manually — works but requires constant manual effort
• Deleting completed tasks — not desirable if a history is needed
• Custom CSS workaround — fragile and depends on internal HTML structure

Additional context

The Deck app already implements a similar concept for archived cards, so there is precedent within the Nextcloud ecosystem for this pattern.
Please use the 👍 reaction if you are affected by the same issue.

The issue names no files or tests. Start by locating the left navigation sidebar and the code that renders completed tasks, then review the Deck app's archived-card behavior as the stated precedent. Done means users can toggle completed-task visibility globally or per list without deleting the tasks or losing their history.