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.

changelog.json

@@ -7,14 +7,16 @@
       "QSO recording works on CW again. Your own sidetone is not in the file, the other station is.",
       "Playing a recording on the air now says why when it cannot — most often no audio output to the radio is configured.",
       "The play-on-air button is hidden on CW: the transmitter builds its tone from the keyer and ignores the audio path. Recording, stopping and resuming stay available.",
-      "A QSO recording no longer comes out empty when one of the two audio sources is silent — the case in CW, where the mic device delivers nothing. The live source is recorded alone."
+      "A QSO recording no longer comes out empty when one of the two audio sources is silent — the case in CW, where the mic device delivers nothing. The live source is recorded alone.",
+      "The recorder now warns on screen within three seconds when a capture device delivers no audio, instead of producing an empty file discovered when the QSO is logged."
     ],
     "fr": [
       "FlexRadio : en split, OpsLog reste sur la slice de réception au lieu de suivre l'émission.",
       "L'enregistrement des QSO fonctionne à nouveau en CW. Votre propre signal d'écoute n'est pas dans le fichier, celui du correspondant l'est.",
       "L'émission d'un enregistrement indique désormais pourquoi elle échoue — le plus souvent aucune sortie audio vers la radio n'est configurée.",
       "Le bouton d'émission de l'enregistrement est masqué en CW : l'émetteur fabrique sa tonalité à partir du manipulateur et ignore la chaîne audio. Enregistrer, arrêter et reprendre restent disponibles.",
-      "Un enregistrement de QSO n'est plus vide quand l'une des deux sources audio est muette — le cas en CW, où le périphérique micro ne débite rien. La source active est enregistrée seule."
+      "Un enregistrement de QSO n'est plus vide quand l'une des deux sources audio est muette — le cas en CW, où le périphérique micro ne débite rien. La source active est enregistrée seule.",
+      "L'enregistreur avertit désormais à l'écran en trois secondes lorsqu'un périphérique de capture ne délivre aucun audio, au lieu de produire un fichier vide découvert à l'enregistrement du QSO."
     ]
   },
   {