Feature: Add support for fullscreen.

[SadMoment]

A simple commit that adds support for fullscreen giving you a more
immersive experience.

To enable fullscreen you would just need to tap on the expand icon next to finger drawing icon on the tool bar

npm test: passed
npm run typecheck: passed

[markm39]

Thanks for the contribution! Please fix the following issues.

• The extra button makes the toolbar too wide on common iPhones, clipping the trailing redo control.
• Status-bar hiding has no cleanup when leaving through another route, such as opening a PDF into the app.