[Feature]: Hide tasks from the sidebar

Problem / Use Case

The sidebar Tasks section shows every task in the workspace, including colleagues' tasks I no longer need to follow. The list gets long and noisy, with no way to trim it for my own view.

Proposed Solution

Add a "Hide task" action to the task context menu (next to "Rename task" and "Manage permissions"). Hiding is a personal, per-user preference: the task stays visible for everyone else and can be shown again from a "Show hidden tasks" toggle or filter.

Alternative Solutions Considered

• Archiving the task, but that affects the whole team and is only appropriate when the task is really done.
• Grouping tasks by directory (#1065) helps organise, but does not remove tasks I don't care about.

Additional Context

Related Issues/PRs:

• #1065
• #1019