fix(service-party-app): joinSession 添加重试逻辑处理乐观锁冲突

问题：
- 多个参与方同时加入会话时会触发乐观锁冲突
- server-party 有重试逻辑可以成功重试
- service-party-app (Electron) 没有重试逻辑，直接失败
- 导致外部参与方无法成功加入 co_managed_keygen 会话

修复：
- joinSession 方法添加最多 3 次重试
- 支持重试的错误类型：optimistic lock、UNAVAILABLE、DEADLINE_EXCEEDED
- 使用指数退避 + 随机抖动避免重试风暴
- 抽取 doJoinSession 内部方法和 sleep 辅助方法

Generated with Claude Code

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

backend/mpc-system/services/service-party-app/electron/modules/grpc-client.ts

@@ -436,13 +436,54 @@ export class GrpcClient extends EventEmitter {
   }
 
   /**
-   * 加入会话
+   * 加入会话（带重试逻辑）
+   *
+   * 支持重试以处理乐观锁冲突等瞬态错误
    */
   async joinSession(sessionId: string, partyId: string, joinToken: string): Promise<JoinSessionResponse> {
     if (!this.client) {
       throw new Error('Not connected');
     }
 
+    const maxRetries = 3;
+    const baseDelayMs = 500;
+    let lastError: Error | null = null;
+
+    for (let attempt = 1; attempt <= maxRetries; attempt++) {
+      try {
+        const result = await this.doJoinSession(sessionId, partyId, joinToken);
+        return result;
+      } catch (err) {
+        lastError = err as Error;
+        const errorMessage = lastError.message || '';
+
+        // 判断是否为可重试的错误
+        const isRetryable =
+          errorMessage.includes('optimistic lock') ||
+          errorMessage.includes('modified by another transaction') ||
+          errorMessage.includes('UNAVAILABLE') ||
+          errorMessage.includes('DEADLINE_EXCEEDED');
+
+        if (!isRetryable || attempt >= maxRetries) {
+          console.error(`[gRPC] joinSession failed after ${attempt} attempt(s):`, errorMessage);
+          throw lastError;
+        }
+
+        // 计算退避延迟（指数退避 + 随机抖动）
+        const delay = baseDelayMs * Math.pow(2, attempt - 1) + Math.random() * 100;
+        console.log(`[gRPC] joinSession attempt ${attempt}/${maxRetries} failed (${errorMessage}), retrying in ${Math.round(delay)}ms...`);
+
+        await this.sleep(delay);
+      }
+    }
+
+    throw lastError || new Error('joinSession failed after retries');
+  }
+
+  /**
+   * 内部方法：执行单次 joinSession 调用
+   */
+  private doJoinSession(sessionId: string, partyId: string, joinToken: string): Promise<JoinSessionResponse> {
     return new Promise((resolve, reject) => {
       (this.client as grpc.Client & { joinSession: (req: unknown, callback: (err: Error | null, res: JoinSessionResponse) => void) => void })
         .joinSession(
@@ -462,6 +503,13 @@ export class GrpcClient extends EventEmitter {
     });
   }
 
+  /**
+   * 辅助方法：异步等待
+   */
+  private sleep(ms: number): Promise<void> {
+    return new Promise(resolve => setTimeout(resolve, ms));
+  }
+
   /**
    * 订阅会话事件（带自动重连）
    */