feat: HAMLOG.online upload and confirmations, Yaesu antenna, a named port holder

HAMLOG.online is a seventh external service: one API key, one ADIF record per
QSO, immediate / delayed / on-close like the rest. They publish no API
documentation, so the protocol is read from THEIR OWN client — the HAMLOG Agent
(github.com/hamlogonline/Agent), which is the authoritative source short of
asking them:

	POST https://hamlog.online/api/agent/
	{"ADIFADD": {"APIKEY": k, "ADIFDATA": record}}   → {"STATUS":"OK"}
	{"KEYSTATUS": {"APIKEY": k}}                     → {"STATUS":"OK","CALLSIGN":…}

Success is STATUS == OK, not "no ERROR field": their failure carries ERROR and
no STATUS, and reading an unknown reply — a proxy page, a maintenance notice —
as an acceptance is how a contact goes missing without anyone noticing.

KEYSTATUS buys something no other service here offers: the key can be checked
BEFORE the first QSO, and the answer names the account. A key pasted from
another callsign is caught in the settings panel rather than through a week of
silent refusals.

Their confirmations are also an award source now, ticked like LoTW rather than
expressed through "custom". It reads the ADIF extras, not a column: the standard
names a field for hamlog.EU and none for hamlog.ONLINE, and borrowing the other
site's field would write a falsehood into every exported log. Three plausible
key names from their own export are accepted too, so nobody has to rename a
column by hand after an export.

Yaesu gains antenna selection (AN), remembered per band — the antenna picked on
a band comes back with it, with no table to fill in anywhere. Rigs with one
socket never answer AN and never show the row; the log says which case it is.

And a serial port that is refused now names its likely holder. OmniRig stays
resident once activated and keeps the port of the rig configured in it, so a
native backend never gets it — "Serial port busy" alone accused nobody, and an
FTDX10 spent a morning being blamed for it.

internal/cat/xiegu.go

@@ -95,7 +95,7 @@ func (x *Xiegu) Connect() error {
 	}
 	p, err := serial.Open(x.portName, &serial.Mode{BaudRate: x.baud})
 	if err != nil {
-		return fmt.Errorf("xiegu: open %s @ %d baud: %w", x.portName, x.baud, err)
+		return fmt.Errorf("xiegu: open %s @ %d baud: %w%s", x.portName, x.baud, err, busyHint(x.portName, err))
 	}
 	p.SetReadTimeout(200 * time.Millisecond)
 	// Deassert DTR and RTS.