Improve dealing with nights

When the computer goes to sleep it still logs some VM as active. This leads to weird results when `this week` is used.

Qubes has a hook for before sleep events. It'd be helpful to write sleep as a special VM, and maybe have a special button/shortcut for explicit time off (register a special event and ignore events during following 5 sec).

Start by reading the Qubes before-sleep hook documentation and tracing where active VM events are logged. Define how sleep should appear as a separate event and how it affects “this week” results. Clarify whether the explicit time-off button or shortcut is also in scope before implementation.

Written by the indexing model from the issue text.