feat: refine month totals and recurring purchase entry

src/export/json.test.ts

@@ -54,6 +54,18 @@ describe("round trip", () => {
 });
 
 describe("validation", () => {
+  test.each(["true", 1, null])("rejects a non-boolean existing-bill entry preference: %s", value => {
+    const data = populated();
+    (data.settings as any).allowExistingRecurringCost = value;
+    expect(() => parseDatasetJson(exportDatasetJson(data))).toThrow(ImportValidationError);
+  });
+
+  test.each([true, false, undefined])("round trips the existing-bill entry preference: %s", value => {
+    const data = populated();
+    data.settings.allowExistingRecurringCost = value;
+    expect(parseDatasetJson(exportDatasetJson(data)).settings.allowExistingRecurringCost).toBe(value);
+  });
+
   test("accepts an unassigned recurring cost and its unassigned charge", () => {
     const data = populated();
     const cost = addRecurringCost(data, {