feat(slice-2): Landing + Sign-in screens, entry routing, API client (§5.1/5.2/6.5)

Two-step email + one-time-code sign-in behind both doors; pure entry-routing rule unit-
tested with Vitest (§6.8); storefront routing reaches a SLICE-2 signed-in placeholder with
sign-out (PUC-9). check.sh now runs the frontend unit tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routing.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from "vitest";
+import { routeFor } from "./routing";
+
+describe("entry routing (SD-0001 §6.5)", () => {
+  it("no session -> landing", () => {
+    expect(routeFor({ account: null, storefront: null })).toBe("landing");
+  });
+
+  it("session without storefront -> create-storefront", () => {
+    expect(routeFor({ account: { email: "m@example.com" }, storefront: null })).toBe(
+      "create-storefront",
+    );
+  });
+
+  it("session with storefront -> admin", () => {
+    expect(
+      routeFor({ account: { email: "m@example.com" }, storefront: { id: 1, name: "Shop" } }),
+    ).toBe("admin");
+  });
+});