fix: name the field when playback has no output device

An operator emptied "Recording mic" and got "no audio output to the radio is
configured", and read it as a restriction: that a recording of the other station
alone cannot be played back. No such rule exists — the mic is an INPUT, one more
source to record, while this message is about the OUTPUT that feeds the
transmitter.

The message now names the setting exactly as the panel labels it, and the log
line carries the other two device values so the next report says which field was
actually empty.

app.go

@@ -7737,8 +7737,13 @@ func (a *App) QSOAudioPlayOnAir() error {
 		// The recorder needs an INPUT from the radio; playing back needs an
 		// OUTPUT into it, which is a different device and may well be unset on a
 		// station that only ever recorded.
-		applog.Printf("qso-rec: play refused — no output device to the radio configured (Settings → Audio)")
+		// Name the FIELD, not the concept. An operator who had just emptied
-		return fmt.Errorf("no audio output to the radio is configured — set it in Settings → Audio")
+		// "Recording mic" read this as "you cannot play back unless you also
+		// record yourself" — a restriction that does not exist. The mic is an
+		// input; this is about the output that feeds the transmitter.
+		applog.Printf("qso-rec: play refused — Settings → Audio → \"To Radio (TX out)\" is empty (from_radio=%q, mic=%q)",
+			cfgEarly.FromRadio, cfgEarly.RecordingDevice)
+		return fmt.Errorf("Settings → Audio → \"To Radio (TX out)\" is empty — that is the output that feeds the transmitter (the recording mic has nothing to do with playing back)")
 	}
 	// A plain WAV in the data dir, overwritten each time: the player takes a
 	// path, and MP3 encoding would add seconds to something the operator is