Add real order persistence, customer accounts, and cart sync

Checkout now persists orders server-side (Payload orders collection,
re-priced from live product data, discount codes redeemed exactly once)
instead of writing a client-only sessionStorage snapshot. Buying requires
an account (registration inline in checkout, no separate step) — accounts
get order history with delivery status, profile/address editing, password
change, and a cart that syncs across devices while logged in.

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

app/lib/cart.ts

@@ -106,3 +106,21 @@ export function useCartCount(): number {
 export function useCart(): CartItem[] {
   return useSyncExternalStore(subscribe, getCart, () => EMPTY_CART);
 }
+
+// Called right after a successful login (LoginForm.tsx, CheckoutContent.tsx's
+// inline login toggle) — folds whatever was saved server-side into the
+// local cart by quantity (addToCart adds to an existing line rather than
+// overwriting it), so items added before logging in aren't lost. CartSync
+// then picks up the resulting change and pushes the merged cart back to
+// the server on its own, closing the loop without a separate save call here.
+export async function mergeServerCartIntoLocal(): Promise<void> {
+  try {
+    const res = await fetch("/api/account/cart");
+    if (!res.ok) return;
+    const data: { cart?: CartItem[] } = await res.json();
+    for (const item of data.cart ?? []) addToCart(item.id, item.qty);
+  } catch {
+    // Best-effort — a failed merge just means the server-side cart stays
+    // as it was; nothing local is lost either way.
+  }
+}