fix(grpc/proxy): persist Private capability on proxy registration

When a proxy connects with capabilities, the registerProxyConnection
path constructs a *proxy.Capabilities literal but only copies three of
the four fields (SupportsCustomPorts, RequireSubdomain, SupportsCrowdsec)
— Private is silently dropped. Effect: the DB's proxies.private column
stays NULL for every connection, GetClusterSupportsPrivate sees no
proxy reported the capability and returns nil, and the cluster API
response strips "private" via openapi omitempty. The dashboard then
can't tell a private cluster apart from a centralised one.

Add the missing field to the struct literal. No schema change — the
Capabilities.Private column already exists (gorm:"embedded"); only
the write path was broken.

After redeploy + a proxy reconnect, /api/reverse-proxies/clusters now
returns "private": true for clusters where an embedded `netbird proxy`
is connected.

management/internals/shared/grpc/proxy.go

@@ -351,6 +351,7 @@ func (s *ProxyServiceServer) registerProxyConnection(ctx context.Context, params
 			SupportsCustomPorts: c.SupportsCustomPorts,
 			RequireSubdomain:    c.RequireSubdomain,
 			SupportsCrowdsec:    c.SupportsCrowdsec,
+			Private:             c.Private,
 		}
 	}