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/screens/SignedIn.tsx

@@ -0,0 +1,34 @@
+// SLICE-2 placeholder for the signed-in surfaces (create-storefront, admin) that land in
+// SLICE-3. It honestly states what's next and makes PUC-9 sign-out reachable now. No
+// fabricated capability (INV-9).
+import { logout } from "../api";
+
+interface Props {
+  email: string;
+  created: boolean;
+  onSignedOut: () => void;
+}
+
+export default function SignedIn({ email, created, onSignedOut }: Props) {
+  async function signOut() {
+    await logout();
+    onSignedOut();
+  }
+
+  return (
+    <main>
+      <header>
+        <strong>ecomm</strong>
+        <span>{email}</span>
+        <button type="button" onClick={signOut}>
+          Sign out
+        </button>
+      </header>
+      <section>
+        <h1>{created ? "Welcome to ecomm" : "Welcome back"}</h1>
+        <p>You're signed in as {email}.</p>
+        <p>Creating your storefront arrives in the next slice. Nothing else is needed yet.</p>
+      </section>
+    </main>
+  );
+}