fix add-to-cart button not submitting in shop mode

The button type condition checked @mode == :live but ThemeHook sets
mode to :shop, so the button rendered as type="button" (doing nothing).
Changed to @mode == :preview to match the existing phx-click pattern.

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

lib/berrypod_web/components/shop_components/product.ex

@@ -1525,7 +1525,7 @@ defmodule BerrypodWeb.ShopComponents.Product do
     ~H"""
     <div class="atc-wrap" data-sticky={to_string(@sticky)}>
       <button
-        type={if @mode == :live, do: "submit", else: "button"}
+        type={if @mode == :preview, do: "button", else: "submit"}
         phx-click={if @mode == :preview, do: open_cart_drawer_js()}
         disabled={@disabled}
         class="atc-btn"