fix(co-sign): fix threshold_n display and add missing fields in GetSignSessionByInviteCode

- Add threshold_n to GetSignSessionByInviteCodeResponse interface
- Fix main.ts to use result.threshold_n instead of result.parties?.length
- Add message_hash, joined_count, join_token to GetSignSessionByInviteCode response
- Generate join token for sign session lookup

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

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

backend/mpc-system/services/account/adapters/input/http/co_managed_handler.go

@@ -685,13 +685,14 @@ func (h *CoManagedHTTPHandler) GetSignSessionByInviteCode(c *gin.Context) {
 	var thresholdN, thresholdT int
 	var status string
 	var expiresAt time.Time
+	var messageHash []byte
 
 	err := h.db.QueryRowContext(ctx, `
 		SELECT id, COALESCE(wallet_name, ''), COALESCE(keygen_session_id::text, ''),
-		       threshold_n, threshold_t, status, expires_at
+		       threshold_n, threshold_t, status, expires_at, COALESCE(message_hash, '')
 		FROM mpc_sessions
 		WHERE invite_code = $1 AND session_type = 'sign'
-	`, inviteCode).Scan(&sessionID, &walletName, &keygenSessionID, &thresholdN, &thresholdT, &status, &expiresAt)
+	`, inviteCode).Scan(&sessionID, &walletName, &keygenSessionID, &thresholdN, &thresholdT, &status, &expiresAt, &messageHash)
 
 	if err != nil {
 		if err == sql.ErrNoRows {
@@ -727,28 +728,48 @@ func (h *CoManagedHTTPHandler) GetSignSessionByInviteCode(c *gin.Context) {
 			"session_id":        sessionID,
 			"keygen_session_id": keygenSessionID,
 			"wallet_name":       walletName,
+			"message_hash":      hex.EncodeToString(messageHash),
 			"threshold_n":       thresholdN,
 			"threshold_t":       thresholdT,
 			"status":            status,
+			"joined_count":      0,
 			"expires_at":        expiresAt.UnixMilli(),
 		})
 		return
 	}
 
+	// Generate join token for this session (wildcard token that allows any party to join)
+	var joinToken string
+	if h.jwtService != nil {
+		sessionUUID, parseErr := uuid.Parse(sessionID)
+		if parseErr == nil {
+			token, err := h.jwtService.GenerateJoinToken(sessionUUID, "*", time.Hour) // Wildcard party ID, 1 hour expiry
+			if err != nil {
+				logger.Warn("Failed to generate join token for sign session",
+					zap.String("session_id", sessionID),
+					zap.Error(err))
+			} else {
+				joinToken = token
+			}
+		}
+	}
+
 	logger.Info("Found sign session for invite_code",
 		zap.String("invite_code", inviteCode),
 		zap.String("session_id", sessionID),
-		zap.String("wallet_name", walletName))
+		zap.String("wallet_name", walletName),
+		zap.Bool("has_join_token", joinToken != ""))
 
 	c.JSON(http.StatusOK, gin.H{
 		"session_id":        sessionID,
 		"keygen_session_id": keygenSessionID,
 		"wallet_name":       walletName,
+		"message_hash":      hex.EncodeToString(messageHash),
 		"threshold_n":       thresholdN,
 		"threshold_t":       thresholdT,
 		"status":            statusResp.Status,
-		"completed_parties": statusResp.CompletedParties,
-		"total_parties":     statusResp.TotalParties,
+		"joined_count":      statusResp.CompletedParties,
 		"expires_at":        expiresAt.UnixMilli(),
+		"join_token":        joinToken,
 	})
 }

backend/mpc-system/services/service-party-app/electron/main.ts

@@ -1712,7 +1712,7 @@ function setupIpcHandlers() {
           messageHash: result.message_hash,
           threshold: {
             t: result.threshold_t,
-            n: result.parties?.length || 0,
+            n: result.threshold_n,
           },
           status: result.status,
           currentParticipants: result.joined_count || 0,

backend/mpc-system/services/service-party-app/electron/modules/account-client.ts

@@ -133,6 +133,7 @@ export interface GetSignSessionByInviteCodeResponse {
   wallet_name: string;
   message_hash: string;
   threshold_t: number;
+  threshold_n: number;
   status: string;
   invite_code: string;
   expires_at: number;