fix: wait for both goroutines before cleanup in HandleConnection

Only waited for one of two copy goroutines, leaking the second. Now waits
for both to complete before closing connections.

Fixes file descriptor leak causing 'too many open files' under load.

Fixes: #56

session/forwarder/forwarder.go

@@ -152,25 +152,26 @@ func (f *Forwarder) HandleConnection(dst io.ReadWriter, src ssh.Channel, remoteA
 
 	log.Printf("Handling new forwarded connection from %s", remoteAddr)
 
-	done := make(chan struct{}, 2)
-
-	go func() {
-		_, err := copyWithBuffer(src, dst)
-		if err != nil && !errors.Is(err, io.EOF) && !errors.Is(err, net.ErrClosed) {
-			log.Printf("Error copying from conn.Reader to channel: %v", err)
-		}
-		done <- struct{}{}
-	}()
+	var wg sync.WaitGroup
+	wg.Add(2)
 
 	go func() {
+		defer wg.Done()
 		_, err := copyWithBuffer(dst, src)
 		if err != nil && !errors.Is(err, io.EOF) && !errors.Is(err, net.ErrClosed) {
-			log.Printf("Error copying from channel to conn.Writer: %v", err)
+			log.Printf("Error copying src→dst: %v", err)
 		}
-		done <- struct{}{}
 	}()
 
-	<-done
+	go func() {
+		defer wg.Done()
+		_, err := copyWithBuffer(src, dst)
+		if err != nil && !errors.Is(err, io.EOF) && !errors.Is(err, net.ErrClosed) {
+			log.Printf("Error copying dst→src: %v", err)
+		}
+	}()
+
+	wg.Wait()
 }
 
 func (f *Forwarder) SetType(tunnelType types.TunnelType) {