add variants to all mock products and fix CSSCache race condition

All 16 mock products now have at least one variant so add-to-cart works
in demo mode. CSSCache.invalidate/0 rescues ArgumentError when the ETS
table doesn't exist yet (seed_defaults runs before CSSCache starts).

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

test/simpleshop_theme_web/live/shop_live/product_show_test.exs

@@ -103,11 +103,11 @@ defmodule SimpleshopThemeWeb.ShopLive.ProductShowTest do
       assert html =~ "disabled"
     end
 
-    test "shows 'One size' for products without options", %{conn: conn} do
-      # Product "2" (Ocean Waves Art Print) has no option_types
+    test "shows single variant for products with one option", %{conn: conn} do
+      # Product "2" (Ocean Waves Art Print) has a single size variant
       {:ok, _view, html} = live(conn, ~p"/products/2")
 
-      assert html =~ "One size"
+      assert html =~ "12×18"
     end
   end