feat(domain): add seed dataset and domain barrel

Add SCHEMA_VERSION and createSeedDataset to initialize first-run state
with three budget envelopes (Video Games, Food, Events and Social),
newId() for generating unique identifiers, and complete the domain
export barrel with all pure domain modules.

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

src/domain/seed.ts

@@ -0,0 +1,38 @@
+import type { Dataset, MonthId, Post } from "./types.ts";
+
+export const SCHEMA_VERSION = 1;
+
+const SEED_POST_NAMES = ["Video Games", "Food", "Events and Social"] as const;
+
+export function newId(): string {
+  return crypto.randomUUID();
+}
+
+/**
+ * The first-run dataset. Takes the start month as an argument because the
+ * domain layer must not read the ambient clock.
+ */
+export function createSeedDataset(startMonth: MonthId): Dataset {
+  const posts: Post[] = SEED_POST_NAMES.map((name, order) => ({
+    id: newId(),
+    name,
+    order,
+    archived: false,
+    currency: "DKK",
+    standingRule: { kind: "fixed", amount: { amount: 0, currency: "DKK" } },
+  }));
+
+  return {
+    settings: {
+      baseCurrency: "DKK",
+      foldStartMonth: startMonth,
+      schemaVersion: SCHEMA_VERSION,
+    },
+    fxRates: [],
+    posts,
+    months: [
+      { id: startMonth, income: { amount: 0, currency: "DKK" }, ruleOverrides: {} },
+    ],
+    purchases: [],
+  };
+}