fix: keep budget sibling order distinct at numeric limit

src/store/actions.test.ts

@@ -2,6 +2,7 @@ import { test, expect, describe } from "bun:test";
 import * as actions from "./actions.ts";
 import { createSeedDataset } from "../domain/seed.ts";
 import { occurrencesByMonth } from "../domain/occurrences.ts";
+import { exportDatasetJson, parseDatasetJson } from "../export/json.ts";
 import type { Dataset } from "../domain/types.ts";
 
 const billInput = (postId: string, folderId: string | null = null) => ({
@@ -83,6 +84,20 @@ describe("budget folder placement", () => {
     const bill = actions.addRecurringCost(data, billInput(post.id));
     expect([folder.order, post.order, bill.order]).toEqual([4, 5, 6]);
   });
+
+  test("adding after the largest finite sibling order stays importable", () => {
+    for (const add of [
+      (data: Dataset) => actions.addBudgetFolder(data, "Utilities", null),
+      (data: Dataset) => actions.addPost(data, "Bills", "DKK"),
+      (data: Dataset) => actions.addRecurringCost(data, billInput(data.posts[0]!.id)),
+    ]) {
+      const data = createSeedDataset("2026-09");
+      data.posts[2]!.order = Number.MAX_VALUE;
+      const added = add(data);
+      expect(added.order).not.toBe(Number.MAX_VALUE);
+      expect(() => parseDatasetJson(exportDatasetJson(data))).not.toThrow();
+    }
+  });
 });
 
 /**

src/store/actions.ts

@@ -64,7 +64,12 @@ export function budgetSiblings(draft: Dataset, parentId: FolderId | null): Budge
 
 function nextSiblingOrder(draft: Dataset, parentId: FolderId | null): number {
   const siblings = siblingRefs(draft, parentId);
-  return siblings.length ? siblings[siblings.length - 1]!.item.order + 1 : 0;
+  if (!siblings.length) return 0;
+  const highest = siblings[siblings.length - 1]!.item.order;
+  const next = highest + 1;
+  if (Number.isFinite(next) && next > highest) return next;
+  siblings.forEach(({ item }, order) => { item.order = order; });
+  return siblings.length;
 }
 
 function requireFolder(draft: Dataset, id: FolderId): BudgetFolder {