Toggle the done mark of a thread from the box list - #169

Before this change, the user had to open a thread to set or remove its done mark, and opening the thread could start a stopped box.

Changes:

• A right click on a thread in the box list sets or removes the done mark at once. On Android, a long press sends the same event.
• iOS sends no context menu event for a long press, so a touch that stays 500 ms on the row also toggles the mark. One press toggles the mark only once, and the thread does not open when the finger comes off.
• The card loads the box list again after the change, so the line-through shows at once.
• The rows have no text selection and no iOS link preview.
• doc/threads.md has one sentence about the right click and the long press.

Tests: a new browser test in dashboard/e2e/threads.test.ts uses a right click and a simulated long press. The long press on a real iPhone and a real Android phone is not tested yet.