feat(slice-3): Create-storefront screen + API helper (§5.3, PUC-4/5/7)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/api.ts

@@ -59,3 +59,21 @@ export async function verifyCode(
 export async function logout(): Promise<void> {
   await fetch("/api/auth/logout", { method: "POST", credentials: "include" });
 }
+
+export interface StorefrontResult {
+  id: number;
+  name: string;
+}
+
+export async function createStorefront(
+  name: string,
+): Promise<{ ok: true; storefront: StorefrontResult } | { ok: false; error: ApiError }> {
+  const resp = await fetch("/api/storefronts", {
+    method: "POST",
+    headers: { "content-type": "application/json" },
+    credentials: "include",
+    body: JSON.stringify(name.trim() ? { name: name.trim() } : {}),
+  });
+  if (resp.ok) return { ok: true, storefront: (await resp.json()) as StorefrontResult };
+  return { ok: false, error: await errorOf(resp) };
+}

frontend/src/screens/CreateStorefront.tsx

@@ -0,0 +1,92 @@
+// Create storefront (SD-0001 §5.3) — a storefront-less Merchant establishes their one
+// storefront (PUC-4/5; PUC-7 defense-in-depth on 409). Visuals per the ui/designs export
+// (hf-storefront). The welcome banner carries PUC-3's honest copy from the verify step.
+import { useState } from "react";
+import { createStorefront, logout, type StorefrontResult } from "../api";
+import { AccountChip, AuthCard, Banner, Eyebrow, Field, Footer, PrimaryButton, Screen, TopBar, Wordmark } from "../ui/kit";
+
+interface Props {
+  email: string;
+  welcome: "new" | "back" | null;
+  onCreated: (storefront: StorefrontResult) => void;
+  onAlreadyOwns: () => void;
+  onSignedOut: () => void;
+}
+
+export default function CreateStorefront({ email, welcome, onCreated, onAlreadyOwns, onSignedOut }: Props) {
+  const [name, setName] = useState("");
+  const [busy, setBusy] = useState(false);
+  const [alreadyOwns, setAlreadyOwns] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  async function signOut() {
+    await logout();
+    onSignedOut();
+  }
+
+  async function submit(e: React.FormEvent) {
+    e.preventDefault();
+    setBusy(true);
+    setError(null);
+    const res = await createStorefront(name);
+    setBusy(false);
+    if (res.ok) {
+      onCreated(res.storefront);
+      return;
+    }
+    if (res.error.code === "already_owns_storefront") setAlreadyOwns(true);
+    else setError(res.error.message);
+  }
+
+  return (
+    <Screen horizon>
+      <TopBar left={<Wordmark size={22} />} right={<AccountChip email={email} onSignOut={signOut} />} />
+      <main className="screen__main">
+        <AuthCard>
+          <form onSubmit={submit} style={{ display: "contents" }}>
+            <div>
+              <Eyebrow>One storefront, fully yours</Eyebrow>
+              <h1 style={{ marginTop: 12 }}>Create your storefront</h1>
+              <p className="card__sub">
+                This is the one thing to do right now. It costs nothing and commits you to nothing.
+              </p>
+            </div>
+            {welcome && (
+              <Banner tone="info" title={welcome === "new" ? "Welcome to ecomm" : "Welcome back"}>
+                {welcome === "new"
+                  ? "A new account was created for this email."
+                  : "Signed in to your existing account."}
+              </Banner>
+            )}
+            {alreadyOwns && (
+              <Banner title="Your account already has its storefront">
+                ecomm is one storefront per account today.{" "}
+                <button type="button" className="lk" onClick={onAlreadyOwns}>
+                  Go to your admin →
+                </button>
+              </Banner>
+            )}
+            {error && <Banner title="That didn't work">{error} Try again.</Banner>}
+            <Field
+              label="Storefront name"
+              optional
+              helper="You can leave this blank — we'll pick a placeholder name you can change any time."
+              inputProps={{
+                type: "text",
+                value: name,
+                placeholder: "e.g. Ben's Bets",
+                autoFocus: true,
+                onChange: (ev) => setName(ev.target.value),
+              }}
+            />
+            <div style={{ display: "flex", flexDirection: "column", gap: 12 }}>
+              <PrimaryButton busy={busy}>{busy ? "Creating…" : "Create storefront"}</PrimaryButton>
+              <p className="note">You can rename, configure, or close your storefront whenever you like.</p>
+            </div>
+          </form>
+        </AuthCard>
+      </main>
+      <Footer />
+    </Screen>
+  );
+}