feat(filter): "is one of" — so a real question can be asked

Every condition was joined by ONE global AND or OR. "2 m or 70 cm, in FT8, since
January" therefore had no expression at all: AND killed the two bands, OR let
every FT8 QSO through. The filter could ask simple questions and nothing else.

Rather than grow nested groups — a tree in the UI to answer something that is
nearly always "this field, any of these values" — the OR lives INSIDE one
condition and everything else keeps ANDing. Two operators, a comma-separated
value, no change to how the rest of the filter behaves.

Two edges that matter more than they look. An empty list matches NOTHING rather
than being dropped: dropping it would widen the result set, the opposite of what
someone typing a filter expects. And "is none of" wraps the column in IFNULL,
because raw SQL NOT IN discards NULL rows — a QSO with no band recorded is not
one of the listed bands, so it belongs in the answer.

internal/qso/qso.go

@@ -1289,6 +1289,23 @@ func columnExpr(field string) (string, bool) {
 }
 
 // conditionSQL turns one condition into a parameterised predicate.
+// splitList parses the comma-separated value of an "in" / "not in" condition.
+//
+// Blanks are dropped, so a trailing comma or a stray space while typing does not
+// silently add an empty value that matches nothing. Case is left alone: the
+// columns this is used on (band, mode, country) are stored in the case the log
+// was written in, and forcing it here would break the ones that are not.
+func splitList(v string) []string {
+	parts := strings.Split(v, ",")
+	out := make([]string, 0, len(parts))
+	for _, p := range parts {
+		if p = strings.TrimSpace(p); p != "" {
+			out = append(out, p)
+		}
+	}
+	return out
+}
+
 func conditionSQL(c Condition) (string, []any, error) {
 	col, ok := columnExpr(c.Field)
 	if !ok {
@@ -1322,6 +1339,36 @@ func conditionSQL(c Condition) (string, []any, error) {
 		return col + " LIKE ?", []any{v + "%"}, nil
 	case "endswith":
 		return col + " LIKE ?", []any{"%" + v}, nil
+	case "in", "notin":
+		// Several values for ONE field, comma-separated.
+		//
+		// This is what makes a real question expressible. The filter joins every
+		// condition with a single AND or OR, so "2 m or 70 cm, in FT8, since
+		// January" could not be asked: AND killed the two bands, OR let every FT8
+		// QSO through. Rather than grow the model nested groups — a tree in the UI
+		// to answer a question that is nearly always "this field, any of these
+		// values" — the OR lives INSIDE one condition and everything else keeps
+		// ANDing.
+		vals := splitList(v)
+		if len(vals) == 0 {
+			// An empty list matches nothing, which is the honest reading. Silently
+			// dropping the condition would widen the result set instead.
+			if c.Op == "in" {
+				return "1=0", nil, nil
+			}
+			return "1=1", nil, nil
+		}
+		ph := strings.TrimSuffix(strings.Repeat("?,", len(vals)), ",")
+		args := make([]any, 0, len(vals))
+		for _, s := range vals {
+			args = append(args, s)
+		}
+		if c.Op == "notin" {
+			// IFNULL, or a NULL column would fail "NOT IN" and vanish from the
+			// result — the row is not one of the listed values, so it belongs.
+			return "IFNULL(" + col + ",'') NOT IN (" + ph + ")", args, nil
+		}
+		return col + " IN (" + ph + ")", args, nil
 	case "empty":
 		return "IFNULL(" + col + ",'') = ''", nil, nil
 	case "notempty":