Make it easier to play snake on the timechain

[casey]

Someone inscribed this nice snake game, but it's hard to play on the /inscription page becuase the arrow keys both control the snake and scroll the page.

Make it so that when an iframe is focused, keyboard events are ignored, the default is prevented. Also indicate visually when the iframe has focus.

[cryptoquick]

Also, someone inscribed Tetris, but you can't see blocks land within the iframe. Zork works great, though.

[cryptoquick]

Some of this is helped by #1250

[PotatoBlood]

press the 'content' link for the snake game: https://ordinals.com/content/6edf80efbbae537b554340c31496439b57bef65357a57f21cbb547bc6287d7bfi0