feat: auto-fallback to default bind IP when invalid bind address detected

When a forward service is updated with a bind IP that doesn't exist on the
host network interfaces, the system now automatically falls back to the
default bind address (listening on all interfaces) instead of failing.

- Added syncForwardServicesWithWarnings to collect fallback warnings
- Implemented fallbackForwardPortToDefaultBind for graceful degradation
- Added UpdateForwardPortBindIP repository method to persist fallback
- Enhanced error detection for 'cannot assign requested address' errors
- Frontend displays warning toasts when fallback occurs
- Added comprehensive unit tests for new error detection functions

go-backend/internal/store/repo/repository_mutations.go

@@ -720,6 +720,18 @@ func (r *Repository) ReplaceForwardPorts(forwardID int64, entries []struct {
 	})
 }
 
+func (r *Repository) UpdateForwardPortBindIP(forwardID, nodeID int64, port int, inIP string) error {
+	if r == nil || r.db == nil {
+		return errors.New("repository not initialized")
+	}
+	if forwardID <= 0 || nodeID <= 0 || port <= 0 {
+		return nil
+	}
+	return r.db.Model(&model.ForwardPort{}).
+		Where("forward_id = ? AND node_id = ? AND port = ?", forwardID, nodeID, port).
+		Update("in_ip", sql.NullString{String: inIP, Valid: strings.TrimSpace(inIP) != ""}).Error
+}
+
 func (r *Repository) RollbackForwardFields(id, userID int64, userName, name string, tunnelID int64, remoteAddr, strategy string, status int, speedID interface{}, now int64) {
 	if r == nil || r.db == nil {
 		return