fix(sign): make password optional for joining sign session

Password field was required to enable the join button, but password
is optional when the share was created without encryption.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

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

@@ -351,7 +351,7 @@ export default function Sign() {
               <button
                 className={styles.primaryButton}
                 onClick={handleJoinSigning}
-                disabled={isLoading || !password}
+                disabled={isLoading}
               >
                 {isLoading ? '加入中...' : '参与签名'}
               </button>