add no-JS progressive enhancement for all shop flows

Every key shop flow now works via plain HTML forms when JS is
unavailable. LiveView progressively enhances when JS connects.

- PDP: form wraps variant/qty/add-to-cart with action="/cart/add"
- Cart page: qty +/- and remove use form POST fallbacks
- Cart/search header icons are now links with phx-click enhancement
- Collection sort form has GET action + noscript submit button
- New /search page with form-based search for no-JS users
- CartController gains add/remove/update_item POST actions
- CartHook gains update_quantity_form and remove_item_form handlers
- Fix flaky analytics tests caused by event table pollution

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

lib/berrypod_web/controllers/cart_controller.ex

@@ -1,9 +1,10 @@
 defmodule BerrypodWeb.CartController do
   @moduledoc """
-  API controller for cart session persistence.
+  Cart controller handling both JSON API persistence and HTML form fallbacks.
 
-  LiveView cannot write to session directly, so cart updates are persisted
-  via this API endpoint called from a JS hook after each cart modification.
+  The JSON `update/2` action is called by a JS hook after each LiveView cart
+  change. The HTML actions (`add/2`, `remove/2`, `update_item/2`) provide
+  no-JS fallbacks via plain form POST + redirect.
   """
 
   use BerrypodWeb, :controller
@@ -11,9 +12,7 @@ defmodule BerrypodWeb.CartController do
   alias Berrypod.Cart
 
   @doc """
-  Updates the cart in session.
-
-  Expects JSON body with `items` as a list of [variant_id, quantity] arrays.
+  Updates the cart in session (JSON API for JS hook).
   """
   def update(conn, %{"items" => items}) when is_list(items) do
     cart_items = Cart.deserialize(items)
@@ -28,4 +27,69 @@ defmodule BerrypodWeb.CartController do
     |> put_status(:bad_request)
     |> json(%{error: "Invalid cart data"})
   end
+
+  @doc """
+  Adds an item to cart via form POST (no-JS fallback).
+  """
+  def add(conn, %{"variant_id" => variant_id, "quantity" => qty_str}) do
+    quantity = parse_quantity(qty_str)
+    cart = Cart.get_from_session(get_session(conn))
+    cart = Cart.add_item(cart, variant_id, quantity)
+
+    conn
+    |> Cart.put_in_session(cart)
+    |> put_flash(:info, "Added to basket")
+    |> redirect(to: ~p"/cart")
+  end
+
+  def add(conn, _params) do
+    conn
+    |> put_flash(:error, "Could not add item to basket")
+    |> redirect(to: ~p"/cart")
+  end
+
+  @doc """
+  Removes an item from cart via form POST (no-JS fallback).
+  """
+  def remove(conn, %{"variant_id" => variant_id}) do
+    cart = Cart.get_from_session(get_session(conn))
+    cart = Cart.remove_item(cart, variant_id)
+
+    conn
+    |> Cart.put_in_session(cart)
+    |> put_flash(:info, "Removed from basket")
+    |> redirect(to: ~p"/cart")
+  end
+
+  @doc """
+  Updates item quantity via form POST (no-JS fallback).
+  """
+  def update_item(conn, %{"variant_id" => variant_id, "quantity" => qty_str}) do
+    quantity = parse_update_quantity(qty_str)
+    cart = Cart.get_from_session(get_session(conn))
+    cart = Cart.update_quantity(cart, variant_id, quantity)
+
+    conn
+    |> Cart.put_in_session(cart)
+    |> redirect(to: ~p"/cart")
+  end
+
+  defp parse_quantity(str) when is_binary(str) do
+    case Integer.parse(str) do
+      {qty, _} when qty > 0 -> qty
+      _ -> 1
+    end
+  end
+
+  defp parse_quantity(_), do: 1
+
+  # Allows 0 and negative values so Cart.update_quantity can remove items
+  defp parse_update_quantity(str) when is_binary(str) do
+    case Integer.parse(str) do
+      {qty, _} -> qty
+      :error -> 1
+    end
+  end
+
+  defp parse_update_quantity(_), do: 1
 end