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/zoom_level.rs

@@ -0,0 +1,50 @@
+use clap::ValueEnum;
+use ratatui::text::Line;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, ValueEnum)]
+pub enum ZoomLevel {
+    ZoomedIn,
+    Default,
+    ZoomedOut,
+}
+
+pub enum CellSizes {
+    Width,
+    Height,
+}
+
+impl ZoomLevel {
+    pub fn get_cell_size(&self, cell_size: CellSizes) -> usize {
+        match (cell_size, self) {
+            (CellSizes::Width, ZoomLevel::ZoomedIn) => 13,
+            (CellSizes::Width, ZoomLevel::Default) => 9,
+            (CellSizes::Width, ZoomLevel::ZoomedOut) => 5,
+            (CellSizes::Height, ZoomLevel::ZoomedIn) => 7,
+            (CellSizes::Height, ZoomLevel::Default) => 5,
+            (CellSizes::Height, ZoomLevel::ZoomedOut) => 3,
+        }
+    }
+
+    pub fn get_cell_text_area(
+        &self,
+        tag: char,
+        structure_level: char,
+        unit: char,
+    ) -> Vec<Line<'_>> {
+        match self {
+            ZoomLevel::ZoomedIn => vec![
+                Line::from(format!(" {}       {} ", structure_level, unit)),
+                Line::from(format!("           ")),
+                Line::from(format!("     {}     ", tag)),
+                Line::from(format!("           ")),
+                Line::from(format!("           ")),
+            ],
+            ZoomLevel::Default => vec![
+                Line::from(format!(" {}   {} ", structure_level, unit)),
+                Line::from(format!("   {}   ", tag)),
+                Line::from(format!("       ")),
+            ],
+            ZoomLevel::ZoomedOut => vec![Line::from(format!(" {} ", tag))],
+        }
+    }
+}