Feature Request: Full screen (or big screen) mode for Slideshow block

### Impacted plugin

Jetpack

### What

A button on the slideshow front-end to activate fullscreen mode

### How

I can imagine a "real" fullscreen mode, but also a "big screen" or "cinema" mode would be great, where the rest of the page is darkened and the slideshow (including control buttons and captions) would be enlarged to maximum size within the browser window.

No file or test is named. Start by locating the Jetpack Slideshow block’s front-end entry point, then clarify whether the request is browser fullscreen or a darkened “big screen” mode; done means a front-end button activates the agreed mode while retaining controls and captions.

Written by the indexing model from the issue text.