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/widgets/board.rs

@@ -7,7 +7,7 @@ use ratatui::{
 use std::rc::Rc;
 
 pub struct BoardWidget<'a> {
-    state: &'a BoardState,
+    state: &'a BoardState<'a>,
 }
 
 impl<'a> BoardWidget<'a> {