refactor: centralize system names, component types, entity types, and animation states into a new Constants module.

src/systems/InputSystem.js

@@ -1,8 +1,9 @@
 import { System } from '../core/System.js';
+import { SystemName } from '../core/Constants.js';
 
 export class InputSystem extends System {
   constructor() {
-    super('InputSystem');
+    super(SystemName.INPUT);
     this.requiredComponents = []; // No required components - handles input globally
     this.priority = 0; // Run first