fix(lotw): show the download moving, retry a busy server, fail a dead one

Three reports of the same shape: a 503 for anything wider than a few days
before, and since the timeout was raised, a window that sits at 'working'
forever. Both are the same missing thing — nothing said what the transfer was
doing.

The body is now read in chunks and every megabyte is reported. A 503/502/504 is
retried twice, 20 s then 40 s, each attempt announced: LoTW answers 'busy' to a
wide report often enough that other loggers simply ask again. And the deadline
is no longer on the whole exchange, which either cut off a healthy slow download
or hid a dead one for twenty minutes — it is ten minutes to START answering
(LoTW builds the whole report first) and two minutes of silence once it has,
which is the difference between slow and dead.

app.go

@@ -12212,7 +12212,10 @@ func (a *App) runDownloadConfirmations(ctx context.Context, svc extsvc.Service,
 			emit(fmt.Sprintf("Downloading all LoTW confirmations for %s…", callLabel))
 		}
 		emit(fmt.Sprintf("Window: since=%q → resolved=%q (scope owncall=%q)", since, sinceDate, ownCall))
-		adifText, err := extsvc.DownloadLoTWConfirmations(ctx, nil, cfg.LoTW, sinceDate, ownCall)
+		// The report arrives over minutes, and a window that says nothing while it
+		// does is indistinguishable from one that has hung — which is what it was
+		// being reported as. Every half-megabyte, say how much has landed.
+		adifText, err := extsvc.DownloadLoTWConfirmations(ctx, nil, cfg.LoTW, sinceDate, ownCall, emit)
 		if err != nil {
 			emit("Download failed: " + err.Error())
 			done(matched, total)