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_enum.rs

@@ -0,0 +1,35 @@
+use crate::app::states::skirmish_states::structures::{BaseBuilding, Stone, Structure, Tunnel};
+use ratatui::style::Color;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Structures {
+    Base(BaseBuilding),
+    Tunnel(Tunnel),
+    Stone(Stone),
+}
+
+impl Structures {
+    pub fn get_color(&self) -> Color {
+        match self {
+            Structures::Base(b) => b.get_color(),
+            Structures::Tunnel(t) => t.get_color(),
+            Structures::Stone(s) => s.get_color(),
+        }
+    }
+
+    pub fn get_tag(&self) -> char {
+        match self {
+            Structures::Base(b) => b.get_tag(),
+            Structures::Tunnel(t) => t.get_tag(),
+            Structures::Stone(s) => s.get_tag(),
+        }
+    }
+
+    pub fn get_level(&self) -> char {
+        match self {
+            Structures::Base(b) => b.get_level(),
+            Structures::Tunnel(t) => t.get_level(),
+            Structures::Stone(s) => s.get_level(),
+        }
+    }
+}