feat: support unassigned purchases and allocation status

Allow month-only purchases to start finance plans. Keep unbudgeted posts distinct from explicit zero rules in overspend reporting.

src/export/json.test.ts

@@ -53,6 +53,11 @@ describe("round trip", () => {
 });
 
 describe("validation", () => {
+  test("accepts a purchase with no post and preserves it through export", () => {
+    const data = populated();
+    data.purchases[0]!.splits = [];
+    expect(parseDatasetJson(exportDatasetJson(data)).purchases[0]!.splits).toEqual([]);
+  });
   test("rejects malformed JSON", () => {
     expect(() => parseDatasetJson("{not json")).toThrow(ImportValidationError);
   });
@@ -89,12 +94,6 @@ describe("validation", () => {
     expect(() => parseDatasetJson(JSON.stringify(data))).toThrow(/exactly one/i);
   });
 
-  test("rejects a purchase with no splits", () => {
-    const data = populated();
-    data.purchases[0]!.splits = [];
-    expect(() => parseDatasetJson(JSON.stringify(data))).toThrow(/at least one split/i);
-  });
-
   test("rejects an unsupported currency", () => {
     const data = populated() as any;
     // CHF, not GBP: GBP is one of the currencies the app ships with now.