refactor: move selection rendering to dedicated UILayer

src/client/InputHandler.ts

@@ -1,5 +1,7 @@
 import { EventBus, GameEvent } from "../core/EventBus";
 import { UserSettings } from "../core/game/UserSettings";
+import { Game } from "../core/game/Game";
+import { UnitView } from "../core/game/GameView";
 
 export class MouseUpEvent implements GameEvent {
   constructor(
@@ -8,6 +10,16 @@ export class MouseUpEvent implements GameEvent {
   ) {}
 }
 
+/**
+ * Event emitted when a unit is selected or deselected
+ */
+export class UnitSelectionEvent implements GameEvent {
+  constructor(
+    public readonly unit: UnitView | null,
+    public readonly isSelected: boolean,
+  ) {}
+}
+
 export class MouseDownEvent implements GameEvent {
   constructor(
     public readonly x: number,