Add guest checkout and admin login link

- Add guest checkout flow: customers can now complete orders without creating an account
  - New /checkout page with login or guest options
  - Guest form collects: full name, email, phone, address
  - Orders track guest info via guestName and guestPhone fields
- Update cart message to reflect guest option
- Add "Admin login" link to customer login page for clear navigation
- Update checkout API to accept and store guest information

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/app/api/checkout/route.ts

@@ -21,6 +21,13 @@ type CheckoutCartItem = {
   placementPreviewUrlBack: string | null;
 };
 
+type GuestInfo = {
+  name: string;
+  email: string;
+  phone: string;
+  address: string;
+};
+
 export async function POST(req: Request) {
   if (!process.env.STRIPE_SECRET_KEY) {
     return NextResponse.json(
@@ -40,6 +47,7 @@ export async function POST(req: Request) {
 
   const body = await req.json();
   const items: CheckoutCartItem[] = body.items ?? [];
+  const guest: GuestInfo | undefined = body.guest;
 
   if (items.length === 0) {
     return NextResponse.json({ error: 'Your bag is empty.' }, { status: 400 });
@@ -79,6 +87,9 @@ export async function POST(req: Request) {
       status: 'PENDING',
       total,
       customerId: customer?.id,
+      email: guest?.email ?? customer?.email,
+      guestName: guest?.name,
+      guestPhone: guest?.phone,
       items: {
         create: pricedItems.map((i) => ({
           productId: i.productId,