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.

app.go

@@ -844,7 +844,11 @@ func (a *App) startup(ctx context.Context) {
 	// Route CAT/OmniRig debug lines into the unified app log (they used to go
 	// to a separate cat.log in the old HamLog folder, which users couldn't find).
 	cat.LogSink = applog.Printf
-	audio.LogSink = applog.Printf  // capture audio-goroutine panics in the app log
+	audio.LogSink = applog.Printf // capture audio-goroutine panics in the app log
+	// A recorder that captures nothing must reach the OPERATOR, not just the log:
+	// they are mid-QSO, and by the time they notice at save time the audio is
+	// gone for good.
+	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
 	db.LogSink = applog.Printf     // which schema migrations ran, and how long they took