fix(lotw): MY_CNTY must not sink a non-US station's upload

TQSL validates MY_CNTY against the ADIF secondary-subdivision list,
which is the US county enumeration — 'XX,County' with a two-letter
state. A Canadian profile produced 'ONTARIO,Kawartha' (the export joined
MY_STATE onto the county wholesale) and TQSL refused the whole record.

Two layers: adifCounty only prefixes a two-letter state, so exports stop
manufacturing the invalid shape; and UploadLoTW scrubs any MY_CNTY that
is not the US shape before signing — MY_STATE and MY_GRIDSQUARE already
locate the station for LoTW, and the US form survives for the county
hunters. Table-tested.

internal/adif/export.go

@@ -443,5 +443,11 @@ func adifCounty(state, county string) string {
 	if c == "" || s == "" || strings.Contains(c, ",") {
 		return c
 	}
+	// The "STATE,County" join is the ADIF secondary-subdivision format, and that
+	// enumeration is a US thing — a two-letter state code. Prefixing a Canadian
+	// "ONTARIO" produced "ONTARIO,Kawartha", which is valid nowhere.
+	if len(s) != 2 {
+		return c
+	}
 	return strings.ToUpper(s) + "," + c
 }