Add close/done button to task detail web page

There is no way to close/complete a task from the web UI task detail page. The status dropdown already lists all statuses including Done, but a prominent "Close" or "Mark Done" button would be more discoverable.

Add a "Close Task" button to the task detail page that sets status to Done.

Implementation:

• In Omni/Task/Web/Pages.hs, in the task detail page (TaskDetailPage instance), add a button near the status badge or in the actions area
• The button should POST to the existing status update endpoint with status=Done
• Use HTMX: hx-post="/tasks/{id}/status" hx-vals='{"status":"Done"}' (or whatever the existing status change endpoint is)
• Style it as a subtle button (not too prominent) since it's a destructive action
• Only show it when status is NOT already Done

Key files: Omni/Task/Web/Pages.hs (TaskDetailPage), Omni/Task/Web/Components.hs (if adding a shared component) Namespace: Omni/Task.hs