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/theme/preview_data.ex

@@ -32,19 +32,25 @@ defmodule SimpleshopTheme.Theme.PreviewData do
 
   @doc """
   Returns cart drawer items formatted for the cart drawer component.
+
+  Format matches Cart.hydrate/1 output for consistency between preview and live modes.
   """
   def cart_drawer_items do
     [
       %{
+        variant_id: "preview-1",
         name: "Mountain Sunrise Art Print",
         variant: "12″ x 18″ / Matte",
-        price: "£24.00",
+        price: 2400,
+        quantity: 1,
         image: "/mockups/mountain-sunrise-print-1.jpg"
       },
       %{
+        variant_id: "preview-2",
         name: "Fern Leaf Mug",
         variant: "11oz / White",
-        price: "£14.99",
+        price: 1499,
+        quantity: 2,
         image: "/mockups/fern-leaf-mug-1.jpg"
       }
     ]