feat: ADIF export field picker — choose exactly which fields to write

Global "Export to ADIF" gains a third option "Choose fields…" next to the
standard-only and full-OpsLog choices, and right-clicking selected QSOs adds
"Export selected — choose fields…". The picker separates official ADIF 3.1.7
fields (grouped by category) from OpsLog/non-standard tags actually present in
the log (discovered via DistinctExtraKeys over extras_json), with All/None per
group and reset-to-defaults; the selection is persisted per user.

Backend: adif.Exporter gains a Fields allow-set that gates every promoted and
extras field in writeRecord; app.go ExportADIF/ExportADIFFiltered/
ExportADIFSelected take a fields []string param, plus new ADIFExtraFields.

frontend/wailsjs/go/main/App.js

@@ -10,6 +10,10 @@ export function ACOMSetPower(arg1) {
   return window['go']['main']['App']['ACOMSetPower'](arg1);
 }
 
+export function ADIFExtraFields() {
+  return window['go']['main']['App']['ADIFExtraFields']();
+}
+
 export function ADIFFields() {
   return window['go']['main']['App']['ADIFFields']();
 }
@@ -306,16 +310,16 @@ export function ExplainAward(arg1, arg2) {
   return window['go']['main']['App']['ExplainAward'](arg1, arg2);
 }
 
-export function ExportADIF(arg1, arg2) {
-  return window['go']['main']['App']['ExportADIF'](arg1, arg2);
+export function ExportADIF(arg1, arg2, arg3) {
+  return window['go']['main']['App']['ExportADIF'](arg1, arg2, arg3);
 }
 
-export function ExportADIFFiltered(arg1, arg2, arg3) {
-  return window['go']['main']['App']['ExportADIFFiltered'](arg1, arg2, arg3);
+export function ExportADIFFiltered(arg1, arg2, arg3, arg4) {
+  return window['go']['main']['App']['ExportADIFFiltered'](arg1, arg2, arg3, arg4);
 }
 
-export function ExportADIFSelected(arg1, arg2, arg3) {
-  return window['go']['main']['App']['ExportADIFSelected'](arg1, arg2, arg3);
+export function ExportADIFSelected(arg1, arg2, arg3, arg4) {
+  return window['go']['main']['App']['ExportADIFSelected'](arg1, arg2, arg3, arg4);
 }
 
 export function ExportAward(arg1) {