UI Suggestions: Hide locked concepts, hot keys and diffing between LHS and RHS

[ichxorya]

Title. I would like to suggest a few UI improvements to the game.

• The player should have the right to hide/unhide the locked concepts (tactics, definitions, theorems...) with a button on each topic. That way, they can focus on what they already have.
• They should also be able to assign hot keys to quickly insert "concepts".
• For the diffing part, there should also be a hide/unhide button to do the trick. The color scheme should be reasonable, too (though I doubt this is a LSP problem - out of scope for this game).

I would like to introduce these with a PR, but unfortunately I'm still new to Lean and couldn't do much. Hope my idea would help!