consolidate shop pages into unified LiveView for editor state persistence

Replace individual shop LiveViews with a single Shop.Page that dispatches
to page modules based on live_action. This enables patch navigation between
pages, preserving socket state (including editor state) across transitions.

Changes:
- Add Shop.Page unified LiveView with handle_params dispatch
- Extract page logic into Shop.Pages.* modules (Home, Product, Collection, etc.)
- Update router to use Shop.Page with live_action for all shop routes
- Change navigate= to patch= in shop component links
- Add maybe_sync_editing_blocks to reload editor state when page changes
- Track editor_page_slug to detect cross-page navigation while editing
- Fix picture element height when hover image disabled
- Extract ThemeEditor components for shared use

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/berrypod_web/live/shop/pages/cart.ex

@@ -0,0 +1,31 @@
+defmodule BerrypodWeb.Shop.Pages.Cart do
+  @moduledoc """
+  Cart page handler for the unified Shop.Page LiveView.
+  """
+
+  import Phoenix.Component, only: [assign: 3]
+
+  alias Berrypod.Pages
+
+  def init(socket, _params, _uri) do
+    page = Pages.get_page("cart")
+
+    socket =
+      socket
+      |> assign(:page_title, "Cart")
+      |> assign(:page, page)
+
+    {:noreply, socket}
+  end
+
+  def handle_params(_params, _uri, socket) do
+    {:noreply, socket}
+  end
+
+  def handle_event(_event, _params, _socket), do: :cont
+
+  # Called from render to compute the subtotal
+  def compute_assigns(assigns) do
+    Map.put(assigns, :cart_page_subtotal, Berrypod.Cart.calculate_subtotal(assigns.cart_items))
+  end
+end