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>

test/berrypod_web/live/shop/cart_test.exs

@@ -68,41 +68,39 @@ defmodule BerrypodWeb.Shop.CartTest do
 
     test "incrementing quantity updates the display", %{
       conn: conn,
-      product: product,
       variant: variant
     } do
       {:ok, view, _html} = conn |> conn_with_cart(variant.id) |> live(~p"/cart")
 
       html =
         view
-        |> element("#main-content button[aria-label='Increase quantity of #{product.title}']")
-        |> render_click()
+        |> form("#main-content form[phx-submit='update_quantity_form']")
+        |> render_submit(%{"quantity" => "2"})
 
       assert html =~ "Quantity updated to 2"
     end
 
     test "decrementing to zero removes the item", %{
       conn: conn,
-      product: product,
       variant: variant
     } do
       {:ok, view, _html} = conn |> conn_with_cart(variant.id) |> live(~p"/cart")
 
       html =
         view
-        |> element("#main-content button[aria-label='Decrease quantity of #{product.title}']")
-        |> render_click()
+        |> form("#main-content form[phx-submit='update_quantity_form']")
+        |> render_submit(%{"quantity" => "0"})
 
       assert html =~ "Your basket is empty"
     end
 
-    test "remove button removes the item", %{conn: conn, product: product, variant: variant} do
+    test "remove button removes the item", %{conn: conn, variant: variant} do
       {:ok, view, _html} = conn |> conn_with_cart(variant.id) |> live(~p"/cart")
 
       html =
         view
-        |> element("#main-content button[aria-label='Remove #{product.title} from cart']")
-        |> render_click()
+        |> form("#main-content form[phx-submit='remove_item_form']")
+        |> render_submit()
 
       assert html =~ "Your basket is empty"
     end