Bug fixes, refactoring and physics migration

This week, I was fixing bugs (there were a lot!) and refactoring.

I finally migrated the game's physics from XPBD to Avian. The migration was seamless since we're only using it for picking, collision checking, and navigation for now 😅

It's worth noting that I also switched from Avian's global Position component to Bevy's relative Transform, as the global Position breaks when there are multiple cities.