chore(battle): initialize result state flag in BattleScene

Adds `_resultShown` property to `BattleScene` constructor, initializing it to `false`. This flag tracks whether the battle result has been displayed and is reset when a new mission begins.

src/scenes/BattleScene.js

@@ -58,6 +58,7 @@ export class BattleScene extends Phaser.Scene {
     this.playerLevel = data.playerLevel || 1;
     this.enemyLevel = data.enemyLevel || 1;
     this.campaignId = data.campaignId || null;
+    this._resultShown = false; // Set this to null when a mission begins.
   }
 
   create() {