diag(audio): say why the Listening device is silent

"I turned the sound back on and nothing comes out of the speakers", with
a log that reports success at every step: the network audio stream up,
664-byte packets arriving, the monitor started. The render goroutine's
error was thrown away — a device unplugged, renamed by Windows or unable
to open at 16 kHz fails exactly there, silently.

It is logged now, with the device id. Two once-only lines either side of
it say whether the decoded audio is reaching a running monitor or
arriving with nobody listening, which separates a device fault from the
speakers simply being switched off.

app.go

@@ -1188,6 +1188,7 @@ func (a *App) startup(ctx context.Context) {
 	audio.AlertSink = func(format string, args ...any) { a.toast(fmt.Sprintf(format, args...)) }
 	extsvc.LogSink = applog.Printf // log raw QRZ (and other) service responses for diagnosis
 	lookup.LogSink = applog.Printf // which call was queried, and why a portable lookup fell back
+	audio.Logf = applog.Printf     // why the Listening device stayed silent, above all
 	db.LogSink = applog.Printf     // which schema migrations ran, and how long they took
 	// Version and executable FIRST, before anything else can fail. Diagnosing a
 	// report means knowing which build produced the log, and that was previously