fix cart hydration for demo mode with mock products

Cart.hydrate/1 now falls back to PreviewData mock products when variant
IDs aren't found in the database, so add-to-cart works on fresh deploys
without synced Printify data.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/simpleshop_theme/cart.ex

@@ -117,11 +117,18 @@ defmodule SimpleshopTheme.Cart do
     else
       variants_map = Products.get_variants_with_products(variant_ids)
 
+      # Fall back to mock data for variant IDs not found in DB (demo mode)
+      missing_ids = variant_ids -- Map.keys(variants_map)
+      mock_map = if missing_ids != [], do: mock_variants_map(missing_ids), else: %{}
+
       cart_items
       |> Enum.map(fn {variant_id, quantity} ->
         case Map.get(variants_map, variant_id) do
           nil ->
-            nil
+            case Map.get(mock_map, variant_id) do
+              nil -> nil
+              item -> %{item | quantity: quantity}
+            end
 
           variant ->
             %{
@@ -162,6 +169,37 @@ defmodule SimpleshopTheme.Cart do
     end
   end
 
+  # Build a lookup map from mock product data for variant IDs not in the DB.
+  # Allows the cart to work in demo mode when no real products are synced.
+  defp mock_variants_map(variant_ids) do
+    ids_set = MapSet.new(variant_ids)
+
+    SimpleshopTheme.Theme.PreviewData.products()
+    |> Enum.flat_map(fn product ->
+      (product[:variants] || [])
+      |> Enum.filter(fn v -> MapSet.member?(ids_set, v.id) end)
+      |> Enum.map(fn v ->
+        image =
+          case product[:image_url] do
+            "/mockups/" <> _ = url -> "#{url}-400.webp"
+            url -> url
+          end
+
+        {v.id,
+         %{
+           variant_id: v.id,
+           product_id: product.id,
+           name: product.name,
+           variant: format_variant_options(v.options),
+           price: v.price,
+           quantity: 1,
+           image: image
+         }}
+      end)
+    end)
+    |> Map.new()
+  end
+
   # =============================================================================
   # Helpers
   # =============================================================================