Add tick thread and Skirmish state update

Introduce `AppEvent::Tick` and a background thread that emits timed
events. `App` now handles `Tick` by calling a new `update` method,
which advances the board and increments a `turn_counter` in
`SkirmishState`. Add required imports, fields, and the
`spawn_tick_thread` call in `main`.

src/app/app.rs

@@ -65,6 +65,7 @@ impl App {
                         .board
                         .change_resize(&window_area, state.skirmish.side_panel);
                 }
+                AppEvent::Tick => self.update()?,
             }
         }
 
@@ -90,4 +91,11 @@ impl App {
         handle_keybindings(self, key_event);
         Ok(())
     }
+
+    fn update(&mut self) -> Result<()> {
+        if let Some(state) = self.states_mut() {
+            state.skirmish.update(&self.args);
+        }
+        Ok(())
+    }
 }