feat: Phase 8 - VFX 和 AI 打赏反应系统

- Unity: 添加 VFXManager 实现金币雨和爱心爆炸特效
- Unity: NetworkManager 支持 GiftEffect 事件
- Unity: AgentVisual 支持自定义时长的 SpeechBubble
- Backend: LLMService 支持生成个性化感谢语
- Backend: Engine 统一处理礼物逻辑 (handle_gift)
- Backend: TwitchBot 接入新的礼物处理流程

unity-client/Assets/Scripts/NetworkManager.cs

@@ -67,6 +67,7 @@ namespace TheIsland.Network
         public event Action<ReviveEventData> OnRevive;
         public event Action<SocialInteractionData> OnSocialInteraction;
         public event Action<WorldStateData> OnWorldUpdate;
+        public event Action<GiftEffectData> OnGiftEffect;  // Phase 8: Gift/Donation effects
         #endregion
 
         #region Private Fields
@@ -343,6 +344,11 @@ namespace TheIsland.Network
                         OnSocialInteraction?.Invoke(socialData);
                         break;
 
+                    case EventTypes.GIFT_EFFECT:
+                        var giftData = JsonUtility.FromJson<GiftEffectData>(dataJson);
+                        OnGiftEffect?.Invoke(giftData);
+                        break;
+
                     case EventTypes.COMMENT:
                         // Comments can be logged but typically not displayed in 3D
                         Debug.Log($"[Chat] {json}");