Add timestamp toggle button in focus mode settings

When I use winterm in my small laptop, I usually to pull the browser to the half of screen, and antoher half of screen is ssh terminal. So the focus mode can make full use of the screen.

Start by locating the focus mode settings and the existing timestamp display in the WindTerm codebase. Confirm how focus mode is enabled, then make the settings control the timestamp visibility; done means the toggle is available and behaves correctly in focus mode.

Written by the indexing model from the issue text.