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/export/json.test.ts

@@ -54,6 +54,24 @@ describe("round trip", () => {
 });
 
 describe("validation", () => {
+  test("accepts an unassigned recurring cost and its unassigned charge", () => {
+    const data = populated();
+    const cost = addRecurringCost(data, {
+      name: "Phone", folderId: null, archived: false,
+      amount: { amount: 30, currency: "DKK" }, startDate: "2026-01",
+      recurrence: { kind: "everyNMonths", n: 1 }, anchoring: "calendar",
+      splitMode: "percent", splits: [],
+    });
+    data.purchases.push({
+      id: "phone-charge", date: "2026-01", description: "Phone",
+      total: { amount: 30, currency: "DKK" }, splitMode: "percent", splits: [],
+      schedule: null, source: { recurringId: cost.id, occurrenceDate: "2026-01" },
+    });
+    const restored = parseDatasetJson(exportDatasetJson(data));
+    expect(restored.recurring[0]!.splits).toEqual([]);
+    expect(restored.purchases.at(-1)!.splits).toEqual([]);
+    expect(() => monthView(restored, "2026-01")).not.toThrow();
+  });
   test("accepts a purchase with no post and preserves it through export", () => {
     const data = populated();
     data.purchases[0]!.splits = [];
@@ -553,8 +571,8 @@ describe("recurring cost validation", () => {
     ).toThrow(/unknown post/);
   });
 
-  test("no splits at all is refused", () => {
-    expect(() => parseDatasetJson(withRecurring([{ ...valid, splits: [] }])))
+  test("a missing splits array is refused", () => {
+    expect(() => parseDatasetJson(withRecurring([{ ...valid, splits: null }])))
       .toThrow(/no splits/);
   });