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/src/components/QSOContextMenu.tsx

@@ -15,6 +15,7 @@ type Props = {
   onSendEQSL?: (ids: number[]) => void;
   onBulkEdit?: (ids: number[]) => void;
   onExportSelected?: (ids: number[]) => void;
+  onExportSelectedFields?: (ids: number[]) => void;
   onExportFiltered?: () => void;
   onExportCabrilloSelected?: (ids: number[]) => void;
   onExportCabrilloFiltered?: () => void;
@@ -35,7 +36,7 @@ const UPLOAD_TARGETS: { service: string; name: string }[] = [
 // or picks a command. (We deliberately do NOT close on scroll/resize: the QSO
 // list auto-refreshes and AG Grid fires internal scroll events on refresh,
 // which used to dismiss the menu the instant it appeared.)
-export function QSOContextMenu({ menu, onClose, onUpdateFromCty, onUpdateFromQRZ, onUpdateFromClublog, onSendTo, onSendRecording, onSendEQSL, onBulkEdit, onExportSelected, onExportFiltered, onExportCabrilloSelected, onExportCabrilloFiltered, onDelete }: Props) {
+export function QSOContextMenu({ menu, onClose, onUpdateFromCty, onUpdateFromQRZ, onUpdateFromClublog, onSendTo, onSendRecording, onSendEQSL, onBulkEdit, onExportSelected, onExportSelectedFields, onExportFiltered, onExportCabrilloSelected, onExportCabrilloFiltered, onDelete }: Props) {
   const { t } = useI18n();
   useEffect(() => {
     if (!menu) return;
@@ -137,6 +138,15 @@ export function QSOContextMenu({ menu, onClose, onUpdateFromCty, onUpdateFromQRZ
               <span>{t('qctx.exportSelectedAdif', { n })}</span>
             </button>
           )}
+          {onExportSelectedFields && (
+            <button
+              className="flex w-full items-center gap-2 px-3 py-1.5 text-left hover:bg-accent/50"
+              onClick={() => { onExportSelectedFields(menu.ids); onClose(); }}
+            >
+              <FileDown className="size-4 text-info" />
+              <span>{t('qctx.exportSelectedFields', { n })}</span>
+            </button>
+          )}
           {onExportFiltered && (
             <button
               className="flex w-full items-center gap-2 px-3 py-1.5 text-left hover:bg-accent/50"