feat: add cart page, cart drawer, and shared cart infrastructure

- Cart context with pure functions for add/remove/update/hydrate
- Price formatting via ex_money (replaces all float division)
- CartHook on_mount with attach_hook for shared event handlers
  (open/close drawer, remove item, PubSub sync)
- Accessible cart drawer with focus trap, scroll lock, aria-live
- Cart page with increment/decrement quantity controls
- Preview mode cart drawer support in theme editor
- Cart persistence to session via JS hook + API endpoint
- 19 tests covering all Cart pure functions

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

lib/simpleshop_theme_web/controllers/cart_controller.ex

@@ -0,0 +1,31 @@
+defmodule SimpleshopThemeWeb.CartController do
+  @moduledoc """
+  API controller for cart session persistence.
+
+  LiveView cannot write to session directly, so cart updates are persisted
+  via this API endpoint called from a JS hook after each cart modification.
+  """
+
+  use SimpleshopThemeWeb, :controller
+
+  alias SimpleshopTheme.Cart
+
+  @doc """
+  Updates the cart in session.
+
+  Expects JSON body with `items` as a list of [variant_id, quantity] arrays.
+  """
+  def update(conn, %{"items" => items}) when is_list(items) do
+    cart_items = Cart.deserialize(items)
+
+    conn
+    |> Cart.put_in_session(cart_items)
+    |> json(%{ok: true})
+  end
+
+  def update(conn, _params) do
+    conn
+    |> put_status(:bad_request)
+    |> json(%{error: "Invalid cart data"})
+  end
+end