Add app icon

[devklick]

The app is currently still using the default icon. The web app deployed to github pages (https://devklick.github.io/basic-blackjack/) still uses the default react icon, and the desktop apps built with electron (https://github.com/devklick/basic-blackjack/releases) still use the default electron icon.

There are icons in src/icons, but they're terrible and need to be binned and properly designed.

The scope of this ticket covers desigining the icon and adding it to both the github pages deployment (should just be a case of replacing the favicon.ico in the public folder), and using the same icon for the electron apps (which is not something I've looked into much, but there's some docs on it here)