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/StandardProductView.tsx

@@ -4,8 +4,10 @@ import { useState } from 'react';
 import { v4 as uuid } from 'uuid';
 import ProductMockup from './ProductMockup';
 import Price from './Price';
+import LoginPromptModal from './LoginPromptModal';
 import { useCart } from '@/lib/cartStore';
 import { getColorName } from '@/lib/colorNames';
+import { checkCustomerAuth } from '@/app/actions';
 import type { ProductDTO } from '@/lib/types';
 
 export default function StandardProductView({ product }: { product: ProductDTO }) {
@@ -14,6 +16,7 @@ export default function StandardProductView({ product }: { product: ProductDTO }
   const [view, setView] = useState<'front' | 'back'>('front');
   const [quantity, setQuantity] = useState(1);
   const [justAdded, setJustAdded] = useState(false);
+  const [showLoginPrompt, setShowLoginPrompt] = useState(false);
   const addItem = useCart((s) => s.addItem);
 
   const hasBack = Boolean(product.imageUrlBack);
@@ -44,7 +47,12 @@ export default function StandardProductView({ product }: { product: ProductDTO }
     return <img src={activePhoto} alt={product.name} className="h-full w-full object-cover" />;
   }
 
-  const handleAddToBag = () => {
+  const handleAddToBag = async () => {
+    const { isAuthenticated } = await checkCustomerAuth();
+    if (!isAuthenticated) {
+      setShowLoginPrompt(true);
+    }
+
     addItem({
       cartItemId: uuid(),
       productId: product.id,
@@ -182,6 +190,8 @@ export default function StandardProductView({ product }: { product: ProductDTO }
         </div>
         {justAdded && <p className="text-sm text-pine">Added to your bag.</p>}
       </div>
+
+      <LoginPromptModal isOpen={showLoginPrompt} onClose={() => setShowLoginPrompt(false)} />
     </div>
   );
 }