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

lib/berrypod_web/live/shop/product_show.ex

@@ -36,10 +36,6 @@ defmodule BerrypodWeb.Shop.ProductShow do
 
         option_types = Product.option_types(product)
         variants = product.variants || []
-        {selected_options, selected_variant} = initialize_variant_selection(variants)
-        available_options = compute_available_options(option_types, variants, selected_options)
-        display_price = variant_price(selected_variant, product)
-        gallery_images = filter_gallery_images(all_images, selected_options["Color"])
 
         if connected?(socket) and socket.assigns[:analytics_visitor_hash] do
           Analytics.track_event(
@@ -62,26 +58,105 @@ defmodule BerrypodWeb.Shop.ProductShow do
           |> assign(:json_ld, product_json_ld(product, og_url, og_image, base))
           |> assign(:product, product)
           |> assign(:all_images, all_images)
-          |> assign(:gallery_images, gallery_images)
           |> assign(:related_products, related_products)
           |> assign(:quantity, 1)
           |> assign(:option_types, option_types)
           |> assign(:variants, variants)
-          |> assign(:selected_options, selected_options)
-          |> assign(:selected_variant, selected_variant)
-          |> assign(:available_options, available_options)
-          |> assign(:display_price, display_price)
 
         {:ok, socket}
     end
   end
 
-  defp initialize_variant_selection([first | _] = _variants) do
-    {first.options, first}
+  @impl true
+  def handle_params(params, _uri, socket) do
+    if socket.assigns[:product] do
+      {:noreply, apply_variant_params(params, socket)}
+    else
+      {:noreply, socket}
+    end
   end
 
-  defp initialize_variant_selection([]) do
-    {%{}, nil}
+  defp apply_variant_params(params, socket) do
+    %{option_types: option_types, variants: variants, product: product, all_images: all_images} =
+      socket.assigns
+
+    option_names = Enum.map(option_types, & &1.name)
+
+    default_options =
+      case variants do
+        [first | _] -> first.options
+        [] -> %{}
+      end
+
+    # Extract option params from the URL, ignoring unknown keys
+    param_options =
+      Enum.reduce(option_names, %{}, fn name, acc ->
+        case params[name] do
+          nil -> acc
+          value -> Map.put(acc, name, value)
+        end
+      end)
+
+    {selected_options, changed_option} =
+      if map_size(param_options) == 0 do
+        {default_options, nil}
+      else
+        merged = Map.merge(default_options, param_options)
+
+        # Identify which option was changed from the default
+        changed =
+          Enum.find(option_names, fn name ->
+            Map.has_key?(param_options, name) and param_options[name] != default_options[name]
+          end)
+
+        {merged, changed}
+      end
+
+    selected_options =
+      if changed_option do
+        resolve_valid_combo(variants, option_types, selected_options, changed_option)
+      else
+        selected_options
+      end
+
+    selected_variant = find_variant(variants, selected_options)
+
+    # If params produced an invalid combo with no matching variant, fall back to defaults
+    {selected_options, selected_variant} =
+      if is_nil(selected_variant) and variants != [] do
+        opts = List.first(variants).options
+        {opts, List.first(variants)}
+      else
+        {selected_options, selected_variant}
+      end
+
+    available_options = compute_available_options(option_types, variants, selected_options)
+    display_price = variant_price(selected_variant, product)
+    gallery_images = filter_gallery_images(all_images, selected_options["Color"])
+
+    option_urls = build_option_urls(option_types, selected_options, product.slug)
+
+    socket
+    |> assign(:selected_options, selected_options)
+    |> assign(:selected_variant, selected_variant)
+    |> assign(:available_options, available_options)
+    |> assign(:display_price, display_price)
+    |> assign(:gallery_images, gallery_images)
+    |> assign(:option_urls, option_urls)
+  end
+
+  defp build_option_urls(option_types, selected_options, slug) do
+    Enum.reduce(option_types, %{}, fn opt_type, acc ->
+      urls =
+        opt_type.values
+        |> Enum.map(fn value ->
+          params = Map.put(selected_options, opt_type.name, value.title)
+          {value.title, ~p"/products/#{slug}?#{params}"}
+        end)
+        |> Map.new()
+
+      Map.put(acc, opt_type.name, urls)
+    end)
   end
 
   defp compute_available_options(option_types, variants, selected_options) do
@@ -148,32 +223,6 @@ defmodule BerrypodWeb.Shop.ProductShow do
     |> Enum.map(& &1.url)
   end
 
-  @impl true
-  def handle_event("select_option", %{"option" => option_name, "selected" => value}, socket) do
-    variants = socket.assigns.variants
-    option_types = socket.assigns.option_types
-
-    selected_options = Map.put(socket.assigns.selected_options, option_name, value)
-    selected_options = resolve_valid_combo(variants, option_types, selected_options, option_name)
-
-    selected_variant = find_variant(variants, selected_options)
-
-    available_options =
-      compute_available_options(option_types, variants, selected_options)
-
-    gallery_images = filter_gallery_images(socket.assigns.all_images, selected_options["Color"])
-
-    socket =
-      socket
-      |> assign(:selected_options, selected_options)
-      |> assign(:selected_variant, selected_variant)
-      |> assign(:available_options, available_options)
-      |> assign(:display_price, variant_price(selected_variant, socket.assigns.product))
-      |> assign(:gallery_images, gallery_images)
-
-    {:noreply, socket}
-  end
-
   @impl true
   def handle_event("increment_quantity", _params, socket) do
     quantity = min(socket.assigns.quantity + 1, 99)