Move ZoomLevel into skirmish_states

Introduce Structure and Unit traits with enum wrappers.
Replace old zoom helper with ZoomLevel methods.
Update imports, BoardState, CellWidget, and CLI to use new locations.

src/app/states/skirmish_states/units/mod.rs

@@ -1,3 +1,7 @@
 mod miner;
+mod units_enum;
+mod units_trait;
 
 pub use miner::MinerUnit;
+pub use units_enum::Units;
+pub use units_trait::{OptionalUnit, Unit};