fix(service-party-app): 修复 participants 为 undefined 导致的崩溃

问题：
- Session.tsx 和 Home.tsx 直接访问 participants.length
- 当后端返回的 session 数据中 participants 为 undefined 时崩溃
- 导致 TypeError: Cannot read properties of undefined (reading length)

修复：
- 添加空值检查 (session.participants || []).length
- 使用 Math.max(0, ...) 防止负数长度

Generated with Claude Code

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

backend/mpc-system/services/service-party-app/src/pages/Home.tsx

@@ -221,7 +221,7 @@ export default function Home() {
                 <div className={styles.infoRow}>
                   <span className={styles.infoLabel}>参与方</span>
                   <span className={styles.infoValue}>
-                    {share.metadata.participants.length} 人
+                    {(share.metadata?.participants || []).length} 人
                   </span>
                 </div>
                 <div className={styles.infoRow}>

backend/mpc-system/services/service-party-app/src/pages/Session.tsx

@@ -187,10 +187,10 @@ export default function Session() {
           {/* 参与方列表 */}
           <div className={styles.section}>
             <h3 className={styles.sectionTitle}>
-              参与方 ({session.participants.length} / {session.threshold.n})
+              参与方 ({(session.participants || []).length} / {session.threshold.n})
             </h3>
             <div className={styles.participantList}>
-              {session.participants.map((participant, index) => (
+              {(session.participants || []).map((participant, index) => (
                 <div key={participant.partyId} className={styles.participant}>
                   <div className={styles.participantInfo}>
                     <span className={styles.participantIndex}>#{index + 1}</span>
@@ -201,10 +201,10 @@ export default function Session() {
                   </span>
                 </div>
               ))}
-              {Array.from({ length: session.threshold.n - session.participants.length }).map((_, index) => (
+              {Array.from({ length: Math.max(0, session.threshold.n - (session.participants || []).length) }).map((_, index) => (
                 <div key={`empty-${index}`} className={`${styles.participant} ${styles.participantEmpty}`}>
                   <div className={styles.participantInfo}>
-                    <span className={styles.participantIndex}>#{session.participants.length + index + 1}</span>
+                    <span className={styles.participantIndex}>#{(session.participants || []).length + index + 1}</span>
                     <span className={styles.participantName}>等待加入...</span>
                   </div>
                   <span className={styles.participantStatus}>⏳</span>