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/extsvc/lotw.go

@@ -11,6 +11,8 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
+	"strconv"
 	"strings"
 	"sync"
 	"syscall"
@@ -337,7 +339,36 @@ func fileExists(p string) bool {
 // they were already uploaded OR outside the callsign certificate's date range.
 // Reporting either as success is how a contact came to be stamped "uploaded"
 // while LoTW had never seen it.
+// scrubMyCnty removes MY_CNTY fields TQSL would refuse. LoTW's secondary
+// subdivisions are the US county enumeration — "XX,County" with a two-letter
+// state — and TQSL rejects the whole record over anything else, so a Canadian
+// station's "ONTARIO,Kawartha" (or a bare county) must simply not be sent.
+// MY_STATE and MY_GRIDSQUARE already locate the station for LoTW.
+var myCntyRe = regexp.MustCompile(`(?i)<MY_CNTY:([0-9]+)(?::[A-Za-z])?>`)
+
+func scrubMyCnty(adif string) string {
+	for {
+		loc := myCntyRe.FindStringSubmatchIndex(adif)
+		if loc == nil {
+			return adif
+		}
+		n, _ := strconv.Atoi(adif[loc[2]:loc[3]])
+		end := loc[1] + n
+		if end > len(adif) {
+			end = len(adif)
+		}
+		val := adif[loc[1]:end]
+		if len(val) > 3 && val[2] == ',' {
+			// "XX,..." — the US shape TQSL accepts; leave it for the county hunters.
+			rest := scrubMyCnty(adif[end:])
+			return adif[:end] + rest
+		}
+		adif = adif[:loc[0]] + strings.TrimLeft(adif[end:], " ")
+	}
+}
+
 func UploadLoTW(ctx context.Context, cfg ServiceConfig, tempDir, adifRecord string) (UploadResult, error) {
+	adifRecord = scrubMyCnty(adifRecord)
 	tqsl := strings.TrimSpace(cfg.TQSLPath)
 	loc := strings.TrimSpace(cfg.StationLocation)
 	switch {