Update formatting to use 4‑space indentation

- Adjust rustfmt.toml to set tab_spaces = 4.
- Reformat imports, struct definitions, function bodies, and match arms
  across the project to match the new style (aligned fields, added
  spacing, etc.).
- Ensure all code complies with the updated rustfmt settings.

src/app/states/skirmish_states/marked_cells.rs

@@ -3,17 +3,17 @@ use std::collections::VecDeque;
 
 #[derive(Debug, Clone, PartialEq)]
 pub struct MarkedCells {
-  pub marking_cells: bool,
-  pub marked_cells: VecDeque<(usize, usize)>,
-  pub selected_unit: Option<Units>,
+    pub marking_cells: bool,
+    pub marked_cells: VecDeque<(usize, usize)>,
+    pub selected_unit: Option<Units>,
 }
 
 impl MarkedCells {
-  pub fn new() -> Self {
-    Self {
-      marking_cells: false,
-      marked_cells: VecDeque::new(),
-      selected_unit: None,
+    pub fn new() -> Self {
+        Self {
+            marking_cells: false,
+            marked_cells: VecDeque::new(),
+            selected_unit: None,
+        }
     }
-  }
 }