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.

internal/qso/qso.go

@@ -366,6 +366,42 @@ func decodeExtras(s string) map[string]string {
 	return m
 }
 
+// DistinctExtraKeys returns the set of keys present across stored QSO extras
+// (the non-promoted ADIF tags kept verbatim), scanning the most recent `sample`
+// rows that have any extras. Capped so it stays fast on a large remote logbook —
+// the extras vocabulary is small and stable, so a recent sample finds it all.
+func (r *Repo) DistinctExtraKeys(ctx context.Context, sample int) ([]string, error) {
+	if sample <= 0 {
+		sample = 5000
+	}
+	rows, err := r.db.QueryContext(ctx,
+		`SELECT extras_json FROM qso
+		 WHERE extras_json IS NOT NULL AND extras_json <> '' AND extras_json <> '{}'
+		 ORDER BY id DESC LIMIT ?`, sample)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	seen := map[string]bool{}
+	for rows.Next() {
+		var s sql.NullString
+		if err := rows.Scan(&s); err != nil {
+			return nil, err
+		}
+		for k := range decodeExtras(s.String) {
+			seen[k] = true
+		}
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	out := make([]string, 0, len(seen))
+	for k := range seen {
+		out = append(out, k)
+	}
+	return out, nil
+}
+
 // Add inserts a QSO and returns its ID.
 func (r *Repo) Add(ctx context.Context, q QSO) (int64, error) {
 	q.Callsign = strings.ToUpper(strings.TrimSpace(q.Callsign))