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/components/page_templates/pdp.html.heex

@@ -32,7 +32,7 @@
             name="variant_id"
             value={@selected_variant && @selected_variant.id}
           />
-          <input type="hidden" name="quantity" value={@quantity} />
+          <%!-- quantity is provided by the quantity_selector input below --%>
 
           <%!-- Dynamic variant selectors --%>
           <%= for option_type <- @option_types do %>
@@ -41,6 +41,7 @@
               selected={@selected_options[option_type.name]}
               available={@available_options[option_type.name] || []}
               mode={@mode}
+              option_urls={(@option_urls || %{})[option_type.name] || %{}}
             />
           <% end %>