feat(settings): a recurring cost's splits are editable

The owner's first report: a cost's post was fixed at creation and could not be
changed. RecurringCost has carried splits and splitMode with the same shape as
Purchase since the first version, so this wires up a capability the data model
already had rather than adding one — and a bill can now span envelopes.

SplitEditor is narrowed from PurchaseDraft to the four fields it actually
reads. PurchaseDraft satisfies that structurally, so the dialog's call site
does not change at all.

The anchoring control is disabled once a cost has charges, matching the store's
refusal rather than offering an action that would fail on save.

Also fixes a carried item: the anchoring handler now expands a month-only
startDate in the same mutate call that switches to lastCharge anchoring,
matching the normalisation the recurrence-kind handler already had — an
everyNMonths cost anchored to lastCharge needs a day-granular startDate, and
the store validates that pairing on every write.

src/ui/purchaseForm.ts

@@ -31,6 +31,19 @@ export interface PurchaseDraft {
   plan: PlanDraft | null;
 }
 
+/**
+ * The slice of a form draft that `SplitEditor` actually reads. `PurchaseDraft`
+ * satisfies it structurally, so the purchase dialog's call site is unchanged —
+ * and a recurring cost can now use the same editor without pretending to be a
+ * purchase.
+ */
+export interface SplitTarget {
+  amount: number;
+  currency: Currency;
+  splitMode: "percent" | "fixed";
+  splits: SplitDraft[];
+}
+
 export function emptyDraft(monthId: MonthId, postId: string): PurchaseDraft {
   return {
     date: monthId,
@@ -45,7 +58,7 @@ export function emptyDraft(monthId: MonthId, postId: string): PurchaseDraft {
 }
 
 /** What the split editor must show: how far the parts are from the whole. */
-export function splitBalance(draft: PurchaseDraft, digits: number): number {
+export function splitBalance(draft: SplitTarget, digits: number): number {
   const target = draft.splitMode === "percent" ? 100 : draft.amount;
   const sum = draft.splits.reduce((total, split) => total + split.value, 0);
   return roundMoney(target - sum, digits);