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/cli.rs

@@ -1,5 +1,5 @@
 use crate::app::{
-    states::{GameMode, PerkDecks, ZoomLevel},
+    states::{GameMode, PerkDecks, skirmish_states::ZoomLevel},
     threads::Soundtrack,
     view::View,
 };