Add login prompt when non-authenticated users add items to cart

- Create LoginPromptModal component for authentication encouragement
- Check customer authentication before adding items to cart
- Show modal for non-logged-in users with options to create account, log in, or continue as guest
- Allow guests to add items (stored in IndexedDB) while prompting to sign up
- Implemented in both StandardProductView (ready-made products) and DesignCanvas (personalized items)
- Add server action checkCustomerAuth() for authentication checks

Addresses user request: prompt guests to create account or log in when adding items to cart.

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

src/components/DesignCanvas.tsx

@@ -6,8 +6,10 @@ import type Konva from 'konva';
 import { v4 as uuid } from 'uuid';
 import ProductMockup from './ProductMockup';
 import Price from './Price';
+import LoginPromptModal from './LoginPromptModal';
 import { useCart } from '@/lib/cartStore';
 import { getEffectivePrice } from '@/lib/pricing';
+import { checkCustomerAuth } from '@/app/actions';
 import type { DesignElement, PrintArea, ProductDTO } from '@/lib/types';
 
 const DISPLAY = 560; // css px size the mockup + stage are rendered at
@@ -310,6 +312,7 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
   const [selectedId, setSelectedId] = useState<string | null>(null);
   const [quantity, setQuantity] = useState(1);
   const [justAdded, setJustAdded] = useState(false);
+  const [showLoginPrompt, setShowLoginPrompt] = useState(false);
   const [nodeReadyTick, setNodeReadyTick] = useState(0);
 
   const stageFrontRef = useRef<Konva.Stage | null>(null);
@@ -462,7 +465,12 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
       ).price
     : product.effectivePrice;
 
-  const handleAddToBag = () => {
+  const handleAddToBag = async () => {
+    const { isAuthenticated } = await checkCustomerAuth();
+    if (!isAuthenticated) {
+      setShowLoginPrompt(true);
+    }
+
     if (!canPersonalize) {
       const flatPhoto = product.colorPhotos[color] ?? product.imageUrl ?? '';
       addItem({
@@ -861,6 +869,8 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
         </div>
         {justAdded && <p className="text-sm text-pine">Added to your bag.</p>}
       </div>
+
+      <LoginPromptModal isOpen={showLoginPrompt} onClose={() => setShowLoginPrompt(false)} />
     </div>
   );
 }