fix(server-party-api): 修复与 pkg/tss 类型不匹配

- TotalParties -> TotalSigners
- result.R/S 从 *big.Int 转换为 []byte
- result.V -> result.RecoveryID

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

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

backend/mpc-system/services/server-party-api/cmd/server/main.go

@@ -598,7 +598,7 @@ func runSigningProtocol(
 	// Create signing config
 	config := tss.SigningConfig{
 		Threshold:    t,
-		TotalParties: n,
+		TotalSigners: n,
 		Timeout:      5 * time.Minute,
 	}
 
@@ -632,7 +632,16 @@ func runSigningProtocol(
 		zap.String("session_id", sessionID.String()),
 		zap.String("party_id", partyID))
 
-	return result.Signature, result.R, result.S, result.V, nil
+	// Convert big.Int to []byte
+	var rBytes, sBytes []byte
+	if result.R != nil {
+		rBytes = result.R.Bytes()
+	}
+	if result.S != nil {
+		sBytes = result.S.Bytes()
+	}
+
+	return result.Signature, rBytes, sBytes, result.RecoveryID, nil
 }
 
 // messageHandler adapts MPCMessage channel to tss.MessageHandler