fix: use _id for MongoDB products in frontend

- SearchView now uses p._id || p.id for product navigation
- ProductResults uses product._id || product.id for React key
- Fixes 'undefined' error when clicking parapharmacy products

apps/frontend/src/components/ProductResults.jsx

@@ -39,7 +39,7 @@ function ProductResults({ products, onSelect }) {
     <div className="product-results">
       {products.map((product) => (
         <ProductCard
-          key={product.id}
+          key={product._id || product.id}
           product={product}
           onSelect={onSelect}
         />

apps/frontend/src/views/SearchView.jsx

@@ -322,10 +322,11 @@ function SearchView({ currentUser, onLoginRequest, initialQuery = '', onNavigate
                 <ProductResults
                   products={products}
                   onSelect={(p) => {
+                    const productId = p._id || p.id;
                     if (onNavigateToProduct) {
-                      onNavigateToProduct(p.source, p.id);
+                      onNavigateToProduct(p.source, productId);
                     } else {
-                      window.location.href = `/product/${p.source}/${p.id}`;
+                      window.location.href = `/product/${p.source}/${productId}`;
                     }
                   }}
                 />