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/tick.rs

@@ -0,0 +1,22 @@
+use crate::app::threads::AppEvent;
+use std::{
+    sync::mpsc::Sender,
+    thread,
+    time::{Duration, Instant},
+};
+
+pub fn spawn_tick_thread(tx: Sender<AppEvent>, interval_ms: u64) {
+    thread::spawn(move || {
+        let interval: Duration = Duration::from_millis(interval_ms);
+        loop {
+            if tx.send(AppEvent::Tick).is_err() {
+                break;
+            }
+
+            let elapsed: Duration = Instant::now().elapsed();
+            if interval > elapsed {
+                thread::sleep(interval - elapsed);
+            }
+        }
+    });
+}