feat: place purchases and bills in budget hierarchy

src/export/json.test.ts

@@ -118,6 +118,21 @@ describe("import rejects invalid folder graph", () => {
   const folder = (id: string, parentId: string | null, order: number) => ({ id, name: id, parentId, order });
   const parse = (data: Dataset) => parseDatasetJson(JSON.stringify(data));
 
+  test("round-trips a standalone purchase and a bill nested beneath a post", () => {
+    const data = populated();
+    data.folders = [folder("home", null, 3)];
+    data.purchases[0]!.placement = { folderId: "home", order: 0 };
+    data.recurring.push({ id: "rent", name: "Rent", order: 0, folderId: null, parentPostId: data.posts[0]!.id, archived: false, amount: { amount: 100, currency: "DKK" }, startDate: "2026-01", recurrence: { kind: "everyNMonths", n: 1 }, anchoring: "calendar", splitMode: "percent", splits: [{ postId: data.posts[0]!.id, value: 100, absorbsRemainder: true }] });
+    const restored = parse(data);
+    expect(restored.purchases[0]!.placement?.folderId).toBe("home");
+    expect(restored.recurring[0]!.parentPostId).toBe(data.posts[0]!.id);
+    restored.recurring[0]!.parentPostId = "missing";
+    expect(() => parse(restored)).toThrow(/parent post/);
+    restored.recurring[0]!.parentPostId = data.posts[0]!.id;
+    restored.purchases[0]!.placement!.folderId = "missing";
+    expect(() => parse(restored)).toThrow(/unknown folder/);
+  });
+
   test("valid nested tree parses", () => {
     const data = populated();
     data.folders = [folder("home", null, 0), folder("bills", "home", 0)];