feat: add month breakdown and session backups

Allow recurring costs and confirmations to remain unassigned when created from a purchase without a post. Add browser backup retention and restore controls, with coverage and documentation updates.

src/ui/components/MonthPieShowcase.test.tsx

@@ -0,0 +1,35 @@
+import { GlobalRegistrator } from "@happy-dom/global-registrator";
+if (!GlobalRegistrator.isRegistered) GlobalRegistrator.register();
+(globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
+
+import { expect, test } from "bun:test";
+import { act } from "react";
+import { createRoot } from "react-dom/client";
+import { createSeedDataset } from "../../domain/seed.ts";
+import { monthView } from "../../domain/views.ts";
+import { store } from "../../store/index.ts";
+import { MonthPieShowcase } from "./MonthPieShowcase.tsx";
+
+test("pie showcase switches between spending and allocations", () => {
+  const data = createSeedDataset("2026-09");
+  data.posts[0]!.rules = [{ from: "2026-09", rule: { kind: "fixed", amount: { amount: 250, currency: "DKK" } } }];
+  data.months[0]!.income = { amount: 1000, currency: "DKK" };
+  data.purchases.push({ id: "p", date: "2026-09", description: "Games", total: { amount: 50, currency: "DKK" }, splitMode: "percent", splits: [{ postId: data.posts[0]!.id, value: 100, absorbsRemainder: true }], schedule: null });
+  const original = { get: store.get, subscribe: store.subscribe };
+  store.get = () => data;
+  store.subscribe = () => () => {};
+  const container = document.createElement("div");
+  const root = createRoot(container);
+  try {
+    act(() => root.render(<MonthPieShowcase view={monthView(data, "2026-09")} baseCurrency="DKK" />));
+    expect(container.textContent).toContain("50.00 DKK");
+    const allocation = [...container.querySelectorAll("button")].find((button) => button.textContent === "Allocated")!;
+    act(() => allocation.click());
+    expect(allocation.getAttribute("aria-pressed")).toBe("true");
+    expect(container.textContent).toContain("250.00 DKK");
+    expect(container.querySelector("svg[role='img']")).not.toBeNull();
+  } finally {
+    act(() => root.unmount());
+    Object.assign(store, original);
+  }
+});