fix: say when a capture device is streaming nothing

Second empty recording from the same station, and this time NEITHER source had
delivered a sample — so the fix for a silent second source could not apply, and
the log still said only "recording was empty" at the end of the QSO.

Two blind spots, both now closed:

  - captureStream's error was discarded. A device that cannot be opened — renamed,
    unplugged, held by another program — looked exactly like one that is merely
    quiet.
  - A WASAPI device can open cleanly and then produce nothing at all. A DAX
    channel with no stream behind it does precisely that, and it is
    indistinguishable from silence until the file turns out to be empty.

A watchdog now reports both, three seconds in, and the first goes to the OPERATOR
as a toast rather than only to the log: they are mid-QSO, and by the time they
find out at save time the audio is gone for good.

internal/audio/recorder.go

@@ -15,6 +15,14 @@ import (
 // Defaults to a no-op so the package is usable without wiring.
 var LogSink = func(string, ...any) {}
 
+// AlertSink receives the few audio problems an operator must see WHILE they are
+// operating, not afterwards in a log file — a capture device that opens but
+// never streams being the one that matters: the recording is silently empty and
+// nothing says so until the QSO is logged and gone.
+//
+// Set to a toast emitter at startup; a no-op keeps the package standalone.
+var AlertSink = func(string, ...any) {}
+
 // recoverGoroutine turns a panic in a long-running audio goroutine into a logged
 // event with a stack trace instead of a silent process-killing crash. (It can't
 // catch a hard Windows access violation from the WASAPI layer — those are fatal
@@ -136,29 +144,63 @@ func (r *Recorder) Start(fromDev, micDev string, prerollSec int) error {
 	go func() {
 		defer r.wg.Done()
 		defer recoverGoroutine("recorder capture (radio)")
-		_ = captureStream(fromDev, stop, func(chunk []byte) {
+		// The error was discarded here. A device that cannot be opened — renamed,
+		// unplugged, held by another program — then looked exactly like a device
+		// that is merely quiet, and the recording came out empty with nothing in
+		// the log to say why.
+		if err := captureStream(fromDev, stop, func(chunk []byte) {
 			s := bytesToInt16(chunk)
 			r.srcMu.Lock()
 			r.bufA = append(r.bufA, s...)
 			r.lastA = time.Now()
 			r.srcMu.Unlock()
-		})
+		}); err != nil {
+			LogSink("recorder: capture from %q failed: %v", fromDev, err)
+		}
 	}()
 	if twoSrc {
 		r.wg.Add(1)
 		go func() {
 			defer r.wg.Done()
 			defer recoverGoroutine("recorder capture (mic)")
-			_ = captureStream(micDev, stop, func(chunk []byte) {
+			if err := captureStream(micDev, stop, func(chunk []byte) {
 				s := bytesToInt16(chunk)
 				r.srcMu.Lock()
 				r.bufB = append(r.bufB, s...)
 				r.lastB = time.Now()
 				r.srcMu.Unlock()
-			})
+			}); err != nil {
+				LogSink("recorder: capture from %q failed: %v", micDev, err)
+			}
 		}()
 	}
 
+	// Watchdog. A WASAPI device can open cleanly and then produce nothing at
+	// all — a DAX channel with no stream behind it does exactly that, and it is
+	// indistinguishable from silence until the recording turns out to be empty
+	// at the end of a QSO. Say it once, three seconds in, while there is still
+	// time to fix the setup.
+	r.wg.Add(1)
+	go func() {
+		defer r.wg.Done()
+		defer recoverGoroutine("recorder watchdog")
+		select {
+		case <-stop:
+			return
+		case <-time.After(3 * time.Second):
+		}
+		r.srcMu.Lock()
+		aQuiet, bQuiet := r.lastA.IsZero(), r.lastB.IsZero()
+		r.srcMu.Unlock()
+		if aQuiet {
+			LogSink("recorder: no audio at all from %q after 3 s — the device opened but nothing is streaming", fromDev)
+			AlertSink("No audio from %s — nothing is being recorded", fromDev)
+		}
+		if twoSrc && bQuiet {
+			LogSink("recorder: no audio at all from %q after 3 s — the device opened but nothing is streaming", micDev)
+		}
+	}()
+
 	// Mixer goroutine.
 	r.wg.Add(1)
 	go func() {