Use Unix sockets for gRPC in mini mode (#8856)

* Use Unix sockets for gRPC between co-located services in mini mode

In `weed mini`, all services run in one process. Previously, inter-service
gRPC traffic (volume↔master, filer↔master, S3↔filer, worker↔admin, etc.)
went through TCP loopback. This adds a gRPC Unix socket registry in the pb
package: mini mode registers a socket path per gRPC port at startup, each
gRPC server additionally listens on its socket, and GrpcDial transparently
routes to the socket via WithContextDialer when a match is found.

Standalone commands (weed master, weed filer, etc.) are unaffected since
no sockets are registered. TCP listeners are kept for external clients.

* Handle Serve error and clean up socket file in ServeGrpcOnLocalSocket

Log non-expected errors from grpcServer.Serve (ignoring
grpc.ErrServerStopped) and always remove the Unix socket file
when Serve returns, ensuring cleanup on Stop/GracefulStop.

weed/command/filer.go

@@ -421,6 +421,7 @@ func (fo *FilerOptions) startFiler() {
 		go grpcS.Serve(grpcLocalL)
 	}
 	go grpcS.Serve(grpcL)
+	pb.ServeGrpcOnLocalSocket(grpcS, grpcPort)
 
 	if runtime.GOOS != "windows" {
 		localSocket := *fo.localSocket