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/state.rs

@@ -8,15 +8,15 @@ use crate::{
 use ratatui::layout::Rect;
 
 #[derive(Debug, Clone, PartialEq)]
-pub struct GameStates {
+pub struct GameStates<'a> {
     pub main_menu: MainMenuState,
-    pub skirmish: SkirmishState,
+    pub skirmish: SkirmishState<'a>,
     pub perk_decks: PerkDecksState,
     pub skills_config: SkillsConfigState,
     pub settings: SettingsState,
 }
 
-impl GameStates {
+impl GameStates<'_> {
     pub fn new(args: &Cli, area: &Rect) -> Self {
         Self {
             main_menu: MainMenuState {