make PDP variant selection work without JS

Variant options (colour, size) are now URL params handled via
handle_params instead of phx-click events. Swatches and size buttons
render as patch links in shop mode, so changing variants works as
plain navigation without JS. Quantity is now a number input that
submits with the form. Unavailable variants render as disabled spans.

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

lib/berrypod_web/live/admin/theme/index.ex

@@ -452,6 +452,7 @@ defmodule BerrypodWeb.Admin.Theme.Index do
       |> assign(:available_options, available_options)
       |> assign(:display_price, display_price)
       |> assign(:quantity, 1)
+      |> assign(:option_urls, %{})
 
     ~H"<BerrypodWeb.PageTemplates.pdp {assigns} />"
   end