fix(account): add message_hash and parties to GetSignSessionByInviteCode

- Query message_hash from mpc_sessions table and return as hex string
- Query participants table to get party_id and party_index
- Add parties array to response for Sign.tsx to use

🤖 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

@@ -615,13 +615,14 @@ 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, 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 {
@@ -684,21 +685,52 @@ func (h *CoManagedHTTPHandler) GetSignSessionByInviteCode(c *gin.Context) {
 		}
 	}
 
+	// Query participants for this session
+	var parties []gin.H
+	partiesRows, err := h.db.QueryContext(ctx, `
+		SELECT party_id, party_index
+		FROM participants
+		WHERE session_id = $1
+		ORDER BY party_index
+	`, sessionID)
+	if err == nil {
+		defer partiesRows.Close()
+		for partiesRows.Next() {
+			var partyID string
+			var partyIndex int
+			if err := partiesRows.Scan(&partyID, &partyIndex); err == nil {
+				parties = append(parties, gin.H{
+					"party_id":    partyID,
+					"party_index": partyIndex,
+				})
+			}
+		}
+	}
+
+	// Convert message_hash to hex string
+	messageHashHex := ""
+	if len(messageHash) > 0 {
+		messageHashHex = hex.EncodeToString(messageHash)
+	}
+
 	logger.Info("Found sign session for invite_code",
 		zap.String("invite_code", inviteCode),
 		zap.String("session_id", sessionID),
 		zap.String("wallet_name", walletName),
-		zap.Bool("has_join_token", joinToken != ""))
+		zap.Bool("has_join_token", joinToken != ""),
+		zap.Int("parties_count", len(parties)))
 
 	c.JSON(http.StatusOK, gin.H{
 		"session_id":        sessionID,
 		"keygen_session_id": keygenSessionID,
 		"wallet_name":       walletName,
+		"message_hash":      messageHashHex,
 		"threshold_n":       thresholdN,
 		"threshold_t":       thresholdT,
 		"status":            statusResp.Status,
 		"completed_parties": statusResp.CompletedParties,
 		"total_parties":     statusResp.TotalParties,
+		"parties":           parties,
 		"expires_at":        expiresAt.UnixMilli(),
 		"join_token":        joinToken,
 	})