Move skirmish Offset and FocusedCell to own module

Extract Offset and FocusedCell structs from skirmish.rs into a new
skirmish_states module. Re-export the structs from the top-level states
module and adjust imports in skirmish.rs.

src/app/states/skirmish_states/focused_cell.rs

@@ -0,0 +1,34 @@
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct FocusedCell {
+    pub row: usize,
+    pub col: usize,
+    pub max_row: usize,
+    pub max_col: usize,
+}
+
+impl FocusedCell {
+    pub fn new(row: usize, col: usize, max_row: usize, max_col: usize) -> Self {
+        Self {
+            row,
+            col,
+            max_row,
+            max_col,
+        }
+    }
+
+    pub fn move_up(&mut self) {
+        self.row = self.row.saturating_sub(1).max(0);
+    }
+
+    pub fn move_down(&mut self) {
+        self.row = self.row.saturating_add(1).min(self.max_row - 1);
+    }
+
+    pub fn move_left(&mut self) {
+        self.col = self.col.saturating_sub(1).max(0);
+    }
+
+    pub fn move_right(&mut self) {
+        self.col = self.col.saturating_add(1).min(self.max_col - 1);
+    }
+}