Add customer accounts: login, signup, reset, activate, orders

- Storefront customer operations in graphql/customer.js and a server-safe
  services/shopify/customer.ts
- Session held in an httpOnly, sameSite=lax cookie set by the
  /api/account handlers; the access token never reaches client JS
- Pages: /account/login, /register, /recover, /reset/[id]/[token] and
  /activate/[id]/[token] for Shopify's emailed links
- /account renders order history as master-detail on one screen, since
  the Storefront API has no standalone order-by-id query for customers
- Header user icon: links to sign-in when signed out, otherwise a menu
  with name, email, order history, and sign out
- Login errors are collapsed and password recovery responds identically
  for known and unknown emails, so neither form enumerates accounts

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jWbNNJLksC1QG8z8845FX

app/api/account/activate/route.ts

@@ -0,0 +1,30 @@
+import {
+  activateAccount,
+  toCustomerGid,
+  customerErrorMessage,
+} from '@/services/shopify/customer';
+import { setSessionToken } from '@/services/shopify/session';
+
+export async function POST(req: Request) {
+  const { id, activationToken, password } = await req.json();
+
+  if (!id || !activationToken || !password) {
+    return Response.json(
+      { error: 'This activation link is incomplete.' },
+      { status: 400 }
+    );
+  }
+
+  const { token, errors } = await activateAccount(
+    toCustomerGid(id),
+    activationToken,
+    password
+  );
+
+  if (!token) {
+    return Response.json({ error: customerErrorMessage(errors) }, { status: 400 });
+  }
+
+  await setSessionToken(token.accessToken, token.expiresAt);
+  return Response.json({ ok: true });
+}