Move buildings and units under skirmish_states

Reorganize the codebase by moving the buildings and units modules
into the skirmish_states namespace, updating imports accordingly.

src/app/states/skirmish.rs

@@ -1,4 +1,4 @@
-use crate::app::{buildings::BaseBuilding, states::skirmish_states::BoardState, units::MinerUnit};
+use crate::app::states::skirmish_states::{BoardState, buildings::BaseBuilding, units::MinerUnit};
 use clap::ValueEnum;
 
 #[derive(Debug, Clone, PartialEq, Eq)]