add denormalized product fields and use Product structs throughout

Adds cheapest_price, compare_at_price, in_stock, on_sale columns to
products table (recomputed from variants after each sync). Shop
components now work with Product structs directly instead of plain
maps from PreviewData. Renames .name to .title, adds Product display
helpers (primary_image, hover_image, option_types) and ProductImage
helpers (display_url, direct_url, source_width). Adds Products context
query functions for storefront use (list_visible_products,
get_visible_product, list_categories with DB-level sort/filter).

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

priv/repo/migrations/20260213005639_add_cached_product_fields.exs

@@ -0,0 +1,12 @@
+defmodule SimpleshopTheme.Repo.Migrations.AddCachedProductFields do
+  use Ecto.Migration
+
+  def change do
+    alter table(:products) do
+      add :cheapest_price, :integer, null: false, default: 0
+      add :compare_at_price, :integer
+      add :in_stock, :boolean, null: false, default: true
+      add :on_sale, :boolean, null: false, default: false
+    end
+  end
+end