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/mod.rs

@@ -3,7 +3,9 @@ mod focused_cell;
 mod offset;
 pub mod structures;
 pub mod units;
+pub mod zoom_level;
 
 pub use board::BoardState;
 pub use focused_cell::{FocusedCell, MoveFocusedCell};
 pub use offset::Offset;
+pub use zoom_level::{CellSizes, ZoomLevel};