fix: 修复出差申请页面头像 key 重复警告

- 审批人头像 key 改为 approver-{id}
- 抄送人头像 key 改为 cc-{id}

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/business-trip/apply.tsx

@@ -509,7 +509,7 @@ export default function BusinessTripApply() {
               {approvers.length > 0 ? (
                 <div className={styles.selectedUsers}>
                   {approvers.map(u => (
-                    <img key={u.id} src={u.avatar || '/default-avatar.png'} alt={u.name} className={styles.avatar} />
+                    <img key={`approver-${u.id}`} src={u.avatar || '/default-avatar.png'} alt={u.name} className={styles.avatar} />
                   ))}
                 </div>
               ) : (
@@ -525,7 +525,7 @@ export default function BusinessTripApply() {
               {ccUsers.length > 0 ? (
                 <div className={styles.selectedUsers}>
                   {ccUsers.map(u => (
-                    <img key={u.id} src={u.avatar || '/default-avatar.png'} alt={u.name} className={styles.avatar} />
+                    <img key={`cc-${u.id}`} src={u.avatar || '/default-avatar.png'} alt={u.name} className={styles.avatar} />
                   ))}
                 </div>
               ) : (