feat: add -auto-transmit flag to start transmitting on connect

When -auto-transmit is passed, barnard automatically keys up the
microphone as soon as the connection sync completes. Useful for
testing, bots, and unattended operation.

Usage:
  barnard -auto-transmit -server=mumble.example.com -username=test_bot

client.go

@@ -110,6 +110,18 @@ func (b *Barnard) OnConnect(e *gumble.ConnectEvent) {
 		b.AddOutputLine(fmt.Sprintf("Welcome message: %s", wmsg))
 	}
 	b.Ui.Refresh()
+
+	// Auto-start transmission if requested
+	if b.AutoTransmit && b.Stream != nil && !b.Tx {
+		err := b.Stream.StartSource(b.UserConfig.GetInputDevice())
+		if err != nil {
+			b.AddOutputLine(fmt.Sprintf("auto-transmit failed: %s", err.Error()))
+		} else {
+			b.Tx = true
+			b.UpdateGeneralStatus(" AutoTx ", true)
+			b.AddOutputLine("Auto-transmit started")
+		}
+	}
 }
 
 func (b *Barnard) OnDisconnect(e *gumble.DisconnectEvent) {