fix(service-party-app): 切换 session 时重新订阅消息流

问题：
- prepareForKeygen 只检查 isPrepared 标志
- 当旧 session 失败后 isPrepared 可能仍为 true
- 新 session 调用 prepareForKeygen 时直接跳过，没有重新订阅
- 导致 external party 仍订阅旧 session 的消息流
- server parties 发送的 TSS 消息无法到达 external party

修复：
- 检查 sessionId 是否变化
- 如果是新 session，先取消旧订阅再重新订阅

Generated with Claude Code

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/services/service-party-app/electron/modules/tss-handler.ts

@@ -123,11 +123,20 @@ export class TSSHandler extends EventEmitter {
    * @param partyId 自己的 party ID
    */
   prepareForKeygen(sessionId: string, partyId: string): void {
-    if (this.isPrepared) {
-      console.log('[TSS] Already prepared for keygen, skip');
+    // 如果已经为同一个 session 准备过，跳过
+    if (this.isPrepared && this.sessionId === sessionId) {
+      console.log('[TSS] Already prepared for same session, skip');
       return;
     }
 
+    // 如果为不同的 session 准备过，先取消旧的订阅
+    if (this.isPrepared && this.sessionId !== sessionId) {
+      console.log(`[TSS] Switching from session ${this.sessionId?.substring(0, 8)}... to ${sessionId.substring(0, 8)}...`);
+      this.grpcClient.removeAllListeners('mpcMessage');
+      this.grpcClient.unsubscribeMessages();
+      this.messageBuffer = [];
+    }
+
     console.log(`[TSS] Preparing for keygen: session=${sessionId.substring(0, 8)}..., party=${partyId.substring(0, 8)}...`);
 
     this.sessionId = sessionId;