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.

app.go

@@ -5465,37 +5465,55 @@ func (a *App) ADIFFields() []adif.FieldDef { return adif.Fields }
 // ADIFVersion returns the ADIF spec version OpsLog conforms to (e.g. "3.1.7").
 func (a *App) ADIFVersion() string { return adif.ADIFVersion() }
 
+// adifFieldSet turns a list of ADIF tag names into an allow-set (uppercased),
+// or nil when the list is empty — nil means "all fields" (standard/full mode),
+// so the two-mode export path is unchanged when no explicit field list is given.
+func adifFieldSet(fields []string) map[string]bool {
+	if len(fields) == 0 {
+		return nil
+	}
+	set := make(map[string]bool, len(fields))
+	for _, f := range fields {
+		if t := strings.ToUpper(strings.TrimSpace(f)); t != "" {
+			set[t] = true
+		}
+	}
+	return set
+}
+
 // ExportADIF writes every QSO to the given file path in ADIF 3.1 format.
 // Streams from DB so memory stays flat even with 100k+ records.
 // includeAppFields=false → portable standard ADIF (for other loggers);
 // true → full export keeping OpsLog/app-specific APP_* fields (round-trip).
-func (a *App) ExportADIF(path string, includeAppFields bool) (adif.ExportResult, error) {
+// fields (optional): when non-empty, export EXACTLY those ADIF tags — the
+// "choose which fields to export" mode (overrides includeAppFields).
+func (a *App) ExportADIF(path string, includeAppFields bool, fields []string) (adif.ExportResult, error) {
 	if a.qso == nil {
 		return adif.ExportResult{}, fmt.Errorf("db not initialized")
 	}
 	if path == "" {
 		return adif.ExportResult{}, fmt.Errorf("empty path")
 	}
-	ex := &adif.Exporter{Repo: a.qso, AppName: "OpsLog", AppVersion: "0.1", IncludeAppFields: includeAppFields}
+	ex := &adif.Exporter{Repo: a.qso, AppName: "OpsLog", AppVersion: "0.1", IncludeAppFields: includeAppFields, Fields: adifFieldSet(fields)}
 	return ex.ExportFile(a.ctx, path)
 }
 
 // ExportADIFFiltered writes the QSOs matching the current filter to path, with
 // NO row limit (the on-screen list is capped by the threshold; this is not).
-func (a *App) ExportADIFFiltered(path string, includeAppFields bool, f qso.QueryFilter) (adif.ExportResult, error) {
+func (a *App) ExportADIFFiltered(path string, includeAppFields bool, f qso.QueryFilter, fields []string) (adif.ExportResult, error) {
 	if a.qso == nil {
 		return adif.ExportResult{}, fmt.Errorf("db not initialized")
 	}
 	if path == "" {
 		return adif.ExportResult{}, fmt.Errorf("empty path")
 	}
-	ex := &adif.Exporter{Repo: a.qso, AppName: "OpsLog", AppVersion: "0.1", IncludeAppFields: includeAppFields}
+	ex := &adif.Exporter{Repo: a.qso, AppName: "OpsLog", AppVersion: "0.1", IncludeAppFields: includeAppFields, Fields: adifFieldSet(fields)}
 	return ex.ExportFileFiltered(a.ctx, path, f)
 }
 
 // ExportADIFSelected writes only the QSOs whose ids are given (the rows the
 // operator highlighted in the grid).
-func (a *App) ExportADIFSelected(path string, includeAppFields bool, ids []int64) (adif.ExportResult, error) {
+func (a *App) ExportADIFSelected(path string, includeAppFields bool, ids []int64, fields []string) (adif.ExportResult, error) {
 	if a.qso == nil {
 		return adif.ExportResult{}, fmt.Errorf("db not initialized")
 	}
@@ -5505,10 +5523,32 @@ func (a *App) ExportADIFSelected(path string, includeAppFields bool, ids []int64
 	if len(ids) == 0 {
 		return adif.ExportResult{}, fmt.Errorf("no QSOs selected")
 	}
-	ex := &adif.Exporter{Repo: a.qso, AppName: "OpsLog", AppVersion: "0.1", IncludeAppFields: includeAppFields}
+	ex := &adif.Exporter{Repo: a.qso, AppName: "OpsLog", AppVersion: "0.1", IncludeAppFields: includeAppFields, Fields: adifFieldSet(fields)}
 	return ex.ExportFileByIDs(a.ctx, path, ids)
 }
 
+// ADIFExtraFields returns the distinct NON-standard ADIF tags actually present
+// in the logbook's stored extras (APP_* and vendor tags) — the "OpsLog / extra
+// fields" group in the export field picker, alongside the static official-ADIF
+// list from ADIFFields(). Sampled + capped so it stays fast on a big remote log.
+func (a *App) ADIFExtraFields() []string {
+	if a.qso == nil {
+		return nil
+	}
+	keys, err := a.qso.DistinctExtraKeys(a.ctx, 5000)
+	if err != nil {
+		return nil
+	}
+	out := make([]string, 0, len(keys))
+	for _, k := range keys {
+		if t := strings.ToUpper(strings.TrimSpace(k)); t != "" && !adif.IsStandardField(t) {
+			out = append(out, t)
+		}
+	}
+	sort.Strings(out)
+	return out
+}
+
 // CabrilloResult reports how many QSOs a Cabrillo export wrote and where.
 type CabrilloResult struct {
 	Count int    `json:"count"`