feat(ui): add finance plan editor

Plans default to an equal split, each slice is individually editable, and
plans compose with splits. Cancelling keeps past slices.

planBalance delegates to domain slicesBalance rather than recomputing the
same total-minus-slices arithmetic, per the project's one-implementation
rule for shared math.

src/ui/components/PurchaseDialog.tsx

@@ -11,6 +11,7 @@ import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
 import { SplitEditor } from "./SplitEditor.tsx";
+import { PlanEditor } from "./PlanEditor.tsx";
 import {
   emptyDraft,
   fromPurchase,
@@ -20,7 +21,7 @@ import {
 } from "../purchaseForm.ts";
 import { useDataset } from "../hooks/useDataset.ts";
 import { useMutate } from "../hooks/useMutate.ts";
-import { addPurchase, updatePurchase } from "../../store/actions.ts";
+import { addPurchase, cancelScheduleFrom, updatePurchase } from "../../store/actions.ts";
 import { CURRENCIES, type MonthId, type Purchase } from "../../domain/types.ts";
 
 interface Props {
@@ -128,6 +129,22 @@ export function PurchaseDialog({ monthId, purchase, trigger }: Props) {
 
           <SplitEditor draft={draft} posts={activePosts} onChange={setDraft} />
 
+          <PlanEditor draft={draft} onChange={setDraft} />
+
+          {purchase?.schedule && !purchase.schedule.cancelledFromMonth && (
+            <Button
+              type="button"
+              variant="outline"
+              size="sm"
+              onClick={() => {
+                mutate((data) => cancelScheduleFrom(data, purchase.id, monthId));
+                setOpen(false);
+              }}
+            >
+              Cancel remaining slices from {monthId}
+            </Button>
+          )}
+
           {errors.length > 0 && (
             <ul className="space-y-1 text-sm text-destructive">
               {errors.map((message) => (