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/threads/events.rs

@@ -5,6 +5,7 @@ use std::sync::mpsc::Sender;
 pub enum AppEvent {
     Input(KeyEvent),
     Resize(u16, u16),
+    Tick,
 }
 
 /// Reads *all* crossterm events and forwards the ones we care about.