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/structures/structures_trait.rs

@@ -0,0 +1,9 @@
+use ratatui::style::Color;
+
+pub trait Structure {
+    fn get_tag(&self) -> char;
+    fn get_color(&self) -> Color;
+    fn get_level(&self) -> char;
+    fn get_stress(&self) -> u8;
+    fn get_durability(&self) -> u16;
+}