Add a first boot preferences dialog for selecting theme/zoom pref

[abose]

• This should only ask a max of 3 questions, in the same window, with reasonable defaults.
• There should only be a done button. No multi-page next-next flows.
• The user should be able to click done and dismiss the dialog with default selections, without doing any selections themselves.

The essentials to ask

• Ask for Zoom preference. [smaller/default/larger] option that previews on hover. dims the dialog. - This as as some users may like bigger text, while some may like smaller code text.
• Ask for dark or light theme preference/use system dark/light mode.