fix(service-party-app): remove password input from Sign page

Current system uses empty password for share encryption, so the password
input field is unnecessary. Removed from both Transfer and Sign pages.

🤖 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

@@ -30,7 +30,8 @@ export default function Sign() {
   const [shares, setShares] = useState<ShareItem[]>([]);
   const [selectedShare, setSelectedShare] = useState<ShareItem | null>(null);
   const [sessionCode, setSessionCode] = useState(urlSessionId || '');
-  const [password, setPassword] = useState('');
+  // 当前系统不使用密码，硬编码为空
+  const password = '';
   const [signingSession, setSigningSession] = useState<SigningSession | null>(null);
   const [signature, setSignature] = useState<string | null>(null);
   const [isLoading, setIsLoading] = useState(false);
@@ -115,11 +116,6 @@ export default function Sign() {
       setError('请选择要使用的钱包份额');
       return;
     }
-    if (!password) {
-      setError('请输入解锁密码');
-      return;
-    }
-
     setIsLoading(true);
     setError(null);
     setStep('signing');
@@ -321,19 +317,6 @@ export default function Sign() {
               </div>
             </div>
 
-            {/* 输入密码 */}
-            <div className={styles.inputGroup}>
-              <label className={styles.label}>解锁密码</label>
-              <input
-                type="password"
-                value={password}
-                onChange={(e) => setPassword(e.target.value)}
-                placeholder="输入您的份额解锁密码"
-                className={styles.input}
-                disabled={isLoading}
-              />
-            </div>
-
             {error && <div className={styles.error}>{error}</div>}
 
             <div className={styles.actions}>
@@ -351,7 +334,7 @@ export default function Sign() {
               <button
                 className={styles.primaryButton}
                 onClick={handleJoinSigning}
-                disabled={isLoading || !password}
+                disabled={isLoading}
               >
                 {isLoading ? '加入中...' : '参与签名'}
               </button>