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/units_enum.rs

@@ -0,0 +1,20 @@
+use crate::app::states::skirmish_states::units::{MinerUnit, OptionalUnit, Unit};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Units {
+    Miner(MinerUnit),
+}
+
+impl Units {
+    pub fn get_tag(&self) -> char {
+        match self {
+            Units::Miner(m) => m.get_tag(),
+        }
+    }
+}
+
+impl OptionalUnit for Option<Units> {
+    fn try_get_tag(&self) -> char {
+        self.map_or(' ', |u| u.get_tag())
+    }
+}