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/RecentQSOsGrid.tsx

@@ -58,6 +58,7 @@ 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;
@@ -262,7 +263,7 @@ export const groupLabel = (t: TFn, g: string): string => t(GRP_KEYS[g] ?? g);
 const stripAwardCols = (st: any[] | null | undefined): any[] =>
   (st ?? []).filter((s) => !String(s?.colId ?? '').startsWith('award_'));
 
-export function RecentQSOsGrid({ rows, selectAllSignal, selectRowSignal, rowDragCall, passOrder, onGridApi, storageKey, onRowDoubleClicked, onRowClicked, onRowSelected, onUpdateFromCty, onUpdateFromQRZ, onUpdateFromClublog, onSendTo, onSendRecording, onSendEQSL, onBulkEdit, onExportSelected, onExportFiltered, onExportCabrilloSelected, onExportCabrilloFiltered, onDelete, onFilteredCountChange, awardCols }: Props) {
+export function RecentQSOsGrid({ rows, selectAllSignal, selectRowSignal, rowDragCall, passOrder, onGridApi, storageKey, onRowDoubleClicked, onRowClicked, onRowSelected, onUpdateFromCty, onUpdateFromQRZ, onUpdateFromClublog, onSendTo, onSendRecording, onSendEQSL, onBulkEdit, onExportSelected, onExportSelectedFields, onExportFiltered, onExportCabrilloSelected, onExportCabrilloFiltered, onDelete, onFilteredCountChange, awardCols }: Props) {
   const { t } = useI18n();
   const gridRef = useRef<any>(null);
   const [pickerOpen, setPickerOpen] = useState(false);
@@ -600,6 +601,7 @@ export function RecentQSOsGrid({ rows, selectAllSignal, selectRowSignal, rowDrag
         onSendEQSL={onSendEQSL}
         onBulkEdit={onBulkEdit}
         onExportSelected={onExportSelected}
+        onExportSelectedFields={onExportSelectedFields}
         onExportFiltered={onExportFiltered}
         onExportCabrilloSelected={onExportCabrilloSelected}
         onExportCabrilloFiltered={onExportCabrilloFiltered}