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/cart_hook.ex

@@ -106,6 +106,34 @@ defmodule BerrypodWeb.CartHook do
     {:halt, socket}
   end
 
+  defp handle_cart_event(
+         "update_quantity_form",
+         %{"variant_id" => id, "quantity" => qty_str},
+         socket
+       ) do
+    quantity = String.to_integer(qty_str)
+    cart = Cart.update_quantity(socket.assigns.raw_cart, id, quantity)
+    new_qty = Cart.get_quantity(cart, id)
+
+    socket =
+      socket
+      |> broadcast_and_update(cart)
+      |> assign(:cart_status, "Quantity updated to #{new_qty}")
+
+    {:halt, socket}
+  end
+
+  defp handle_cart_event("remove_item_form", %{"variant_id" => id}, socket) do
+    cart = Cart.remove_item(socket.assigns.raw_cart, id)
+
+    socket =
+      socket
+      |> broadcast_and_update(cart)
+      |> assign(:cart_status, "Item removed from cart")
+
+    {:halt, socket}
+  end
+
   defp handle_cart_event(_event, _params, socket), do: {:cont, socket}
 
   # Shared info handlers