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/live/theme_live/index.ex

@@ -33,6 +33,7 @@ defmodule SimpleshopThemeWeb.ThemeLive.Index do
       |> assign(:header_image, header_image)
       |> assign(:customise_open, false)
       |> assign(:sidebar_collapsed, false)
+      |> assign(:cart_drawer_open, false)
       |> allow_upload(:logo_upload,
         accept: ~w(.png .jpg .jpeg .webp .svg),
         max_entries: 1,
@@ -287,6 +288,16 @@ defmodule SimpleshopThemeWeb.ThemeLive.Index do
     {:noreply, assign(socket, :sidebar_collapsed, !socket.assigns.sidebar_collapsed)}
   end
 
+  @impl true
+  def handle_event("open_cart_drawer", _params, socket) do
+    {:noreply, assign(socket, :cart_drawer_open, true)}
+  end
+
+  @impl true
+  def handle_event("close_cart_drawer", _params, socket) do
+    {:noreply, assign(socket, :cart_drawer_open, false)}
+  end
+
   @impl true
   def handle_event("noop", _params, socket) do
     {:noreply, socket}
@@ -303,6 +314,7 @@ defmodule SimpleshopThemeWeb.ThemeLive.Index do
   attr :theme_settings, :map, required: true
   attr :logo_image, :any, required: true
   attr :header_image, :any, required: true
+  attr :cart_drawer_open, :boolean, default: false
 
   defp preview_page(%{page: :home} = assigns) do
     ~H"""
@@ -315,6 +327,7 @@ defmodule SimpleshopThemeWeb.ThemeLive.Index do
       cart_items={PreviewData.cart_drawer_items()}
       cart_count={2}
       cart_subtotal="£72.00"
+      cart_drawer_open={@cart_drawer_open}
     />
     """
   end
@@ -330,6 +343,7 @@ defmodule SimpleshopThemeWeb.ThemeLive.Index do
       cart_items={PreviewData.cart_drawer_items()}
       cart_count={2}
       cart_subtotal="£72.00"
+      cart_drawer_open={@cart_drawer_open}
     />
     """
   end
@@ -379,6 +393,7 @@ defmodule SimpleshopThemeWeb.ThemeLive.Index do
       cart_items={PreviewData.cart_drawer_items()}
       cart_count={2}
       cart_subtotal="£72.00"
+      cart_drawer_open={@cart_drawer_open}
       option_types={@option_types}
       selected_options={@selected_options}
       available_options={@available_options}
@@ -409,6 +424,7 @@ defmodule SimpleshopThemeWeb.ThemeLive.Index do
       cart_items={PreviewData.cart_drawer_items()}
       cart_count={2}
       cart_subtotal="£72.00"
+      cart_drawer_open={@cart_drawer_open}
     />
     """
   end
@@ -423,6 +439,7 @@ defmodule SimpleshopThemeWeb.ThemeLive.Index do
       cart_items={PreviewData.cart_drawer_items()}
       cart_count={2}
       cart_subtotal="£72.00"
+      cart_drawer_open={@cart_drawer_open}
     />
     """
   end
@@ -437,6 +454,7 @@ defmodule SimpleshopThemeWeb.ThemeLive.Index do
       cart_items={PreviewData.cart_drawer_items()}
       cart_count={2}
       cart_subtotal="£72.00"
+      cart_drawer_open={@cart_drawer_open}
     />
     """
   end
@@ -455,6 +473,7 @@ defmodule SimpleshopThemeWeb.ThemeLive.Index do
       cart_items={PreviewData.cart_drawer_items()}
       cart_count={2}
       cart_subtotal="£72.00"
+      cart_drawer_open={@cart_drawer_open}
     />
     """
   end

lib/simpleshop_theme_web/live/theme_live/index.html.heex

@@ -1164,6 +1164,7 @@
             theme_settings={@theme_settings}
             logo_image={@logo_image}
             header_image={@header_image}
+            cart_drawer_open={@cart_drawer_open}
           />
         </div>
       </div>