feat(ui): add purchase dialog with split editor

Validation lives in purchaseForm.ts as pure functions. Unbalanced splits
and over-budget purchases are deliberately not errors.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/ui/purchaseForm.ts

@@ -0,0 +1,115 @@
+import { roundMoney } from "../domain/money.ts";
+import type {
+  Currency,
+  IsoDate,
+  MonthId,
+  Purchase,
+  ScheduleSlice,
+} from "../domain/types.ts";
+
+export interface SplitDraft {
+  postId: string;
+  value: number;
+  absorbsRemainder: boolean;
+}
+
+export interface PlanDraft {
+  startMonth: MonthId;
+  slices: ScheduleSlice[];
+  cancelledFromMonth?: MonthId;
+}
+
+export interface PurchaseDraft {
+  date: IsoDate;
+  description: string;
+  amount: number;
+  currency: Currency;
+  splitMode: "percent" | "fixed";
+  splits: SplitDraft[];
+  plan: PlanDraft | null;
+}
+
+export function emptyDraft(monthId: MonthId, postId: string): PurchaseDraft {
+  return {
+    date: `${monthId}-01`,
+    description: "",
+    amount: 0,
+    currency: "DKK",
+    splitMode: "percent",
+    splits: [{ postId, value: 100, absorbsRemainder: true }],
+    plan: null,
+  };
+}
+
+/** What the split editor must show: how far the parts are from the whole. */
+export function splitBalance(draft: PurchaseDraft): number {
+  const target = draft.splitMode === "percent" ? 100 : draft.amount;
+  const sum = draft.splits.reduce((total, split) => total + split.value, 0);
+  return roundMoney(target - sum, draft.currency);
+}
+
+export function validatePurchase(draft: PurchaseDraft): string[] {
+  const errors: string[] = [];
+
+  if (draft.description.trim() === "") {
+    errors.push("Give the purchase a description.");
+  }
+  if (draft.amount === 0) {
+    errors.push("Enter an amount other than zero.");
+  }
+  if (draft.splits.length === 0) {
+    errors.push("Add at least one post to split across.");
+  }
+  if (draft.splits.filter((s) => s.absorbsRemainder).length !== 1) {
+    errors.push("Choose exactly one post to absorb rounding.");
+  }
+  const postIds = draft.splits.map((s) => s.postId);
+  if (new Set(postIds).size !== postIds.length) {
+    errors.push("Each post can only appear once.");
+  }
+  if (postIds.some((id) => id === "")) {
+    errors.push("Choose a post for every split.");
+  }
+
+  // Deliberately absent: any check on whether splits balance, or whether the
+  // purchase pushes a post over budget. Both are permitted by design.
+  return errors;
+}
+
+export function toPurchase(draft: PurchaseDraft): Omit<Purchase, "id"> {
+  return {
+    date: draft.date,
+    description: draft.description.trim(),
+    total: { amount: draft.amount, currency: draft.currency },
+    splitMode: draft.splitMode,
+    splits: draft.splits.map((s) => ({ ...s })),
+    schedule: draft.plan
+      ? {
+          slices: draft.plan.slices.map((s) => ({ ...s, amount: { ...s.amount } })),
+          ...(draft.plan.cancelledFromMonth
+            ? { cancelledFromMonth: draft.plan.cancelledFromMonth }
+            : {}),
+        }
+      : null,
+  };
+}
+
+export function fromPurchase(purchase: Purchase): PurchaseDraft {
+  return {
+    date: purchase.date,
+    description: purchase.description,
+    amount: purchase.total.amount,
+    currency: purchase.total.currency,
+    splitMode: purchase.splitMode,
+    splits: purchase.splits.map((s) => ({ ...s })),
+    plan: purchase.schedule
+      ? {
+          startMonth: purchase.schedule.slices[0]?.month ?? "",
+          slices: purchase.schedule.slices.map((s) => ({ ...s, amount: { ...s.amount } })),
+          ...(purchase.schedule.cancelledFromMonth
+            ? { cancelledFromMonth: purchase.schedule.cancelledFromMonth }
+            : {}),
+        }
+      : null,
+  };
+}