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/Landing.tsx

@@ -0,0 +1,27 @@
+// Landing (SD-0001 §5.1) — a Visitor learns what ecomm is and picks a door. Honest voice,
+// no trial/pricing/fee (corpus 14.01.0011). Both doors open the same sign-in flow (§5.2);
+// they differ only in framing.
+interface Props {
+  onGetStarted: () => void;
+  onLogIn: () => void;
+}
+
+export default function Landing({ onGetStarted, onLogIn }: Props) {
+  return (
+    <main>
+      <header>
+        <strong>ecomm</strong>
+        <button type="button" onClick={onLogIn}>
+          Log in
+        </button>
+      </header>
+      <section>
+        <h1>Honest commerce. Your storefront is yours.</h1>
+        <p>Sell online on a platform that treats you straight — no dark patterns, no lock-in.</p>
+        <button type="button" onClick={onGetStarted}>
+          Create your storefront
+        </button>
+      </section>
+    </main>
+  );
+}