Add lifetime parameter and MarkedCells struct

Refactor App, GameStates, SkirmishState and BoardState to carry a
generic
lifetime `'a`. Introduce a new `MarkedCells` type that holds marking
state
and selected unit, replacing the previous `marking_cells` flag and raw
deque. Update related methods, keybindings, view implementations and
module exports to use the new structures.

src/app/views/main_menu.rs

@@ -33,7 +33,9 @@ fn main_menu_layout(area: Rect) -> [Rect; 2] {
 }
 
 pub fn main_menu_view(app: &App, area: Rect, buf: &mut Buffer) {
-    let Some(states) = app.states() else { return };
+    let Some(states) = app.states_ref() else {
+        return;
+    };
 
     let [main_menu_area, keybindings_area] = main_menu_layout(area);