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/controllers/cart_controller_test.exs

@@ -0,0 +1,111 @@
+defmodule BerrypodWeb.CartControllerTest do
+  use BerrypodWeb.ConnCase, async: false
+
+  import Berrypod.AccountsFixtures
+
+  alias Berrypod.ProductsFixtures
+
+  setup do
+    user_fixture()
+    {:ok, _} = Berrypod.Settings.set_site_live(true)
+    :ok
+  end
+
+  defp create_variant(_context) do
+    product = ProductsFixtures.complete_product_fixture(%{title: "Test Print"})
+    variant = List.first(product.variants)
+    %{product: product, variant: variant}
+  end
+
+  defp conn_with_cart(conn, variant_id, qty) do
+    Phoenix.ConnTest.init_test_session(conn, %{"cart" => [{variant_id, qty}]})
+  end
+
+  describe "POST /cart/add" do
+    setup [:create_variant]
+
+    test "adds item to session cart and redirects to /cart", %{conn: conn, variant: variant} do
+      conn = post(conn, ~p"/cart/add", %{"variant_id" => variant.id, "quantity" => "1"})
+
+      assert redirected_to(conn) == "/cart"
+      assert Phoenix.Flash.get(conn.assigns.flash, :info) == "Added to basket"
+
+      cart = get_session(conn, "cart")
+      assert [{variant_id, 1}] = cart
+      assert variant_id == variant.id
+    end
+
+    test "increments quantity if item already in cart", %{conn: conn, variant: variant} do
+      conn =
+        conn
+        |> conn_with_cart(variant.id, 2)
+        |> post(~p"/cart/add", %{"variant_id" => variant.id, "quantity" => "3"})
+
+      cart = get_session(conn, "cart")
+      assert [{_, 5}] = cart
+    end
+
+    test "defaults quantity to 1 for invalid values", %{conn: conn, variant: variant} do
+      conn = post(conn, ~p"/cart/add", %{"variant_id" => variant.id, "quantity" => "abc"})
+
+      cart = get_session(conn, "cart")
+      assert [{_, 1}] = cart
+    end
+
+    test "handles missing params gracefully", %{conn: conn} do
+      conn = post(conn, ~p"/cart/add", %{})
+
+      assert redirected_to(conn) == "/cart"
+      assert Phoenix.Flash.get(conn.assigns.flash, :error) == "Could not add item to basket"
+    end
+  end
+
+  describe "POST /cart/remove" do
+    setup [:create_variant]
+
+    test "removes item from session cart", %{conn: conn, variant: variant} do
+      conn =
+        conn
+        |> conn_with_cart(variant.id, 2)
+        |> post(~p"/cart/remove", %{"variant_id" => variant.id})
+
+      assert redirected_to(conn) == "/cart"
+      assert Phoenix.Flash.get(conn.assigns.flash, :info) == "Removed from basket"
+
+      cart = get_session(conn, "cart")
+      assert cart == []
+    end
+
+    test "handles removing non-existent item", %{conn: conn} do
+      conn = post(conn, ~p"/cart/remove", %{"variant_id" => Ecto.UUID.generate()})
+
+      assert redirected_to(conn) == "/cart"
+    end
+  end
+
+  describe "POST /cart/update" do
+    setup [:create_variant]
+
+    test "updates quantity in session cart", %{conn: conn, variant: variant} do
+      conn =
+        conn
+        |> conn_with_cart(variant.id, 1)
+        |> post(~p"/cart/update", %{"variant_id" => variant.id, "quantity" => "5"})
+
+      assert redirected_to(conn) == "/cart"
+
+      cart = get_session(conn, "cart")
+      assert [{_, 5}] = cart
+    end
+
+    test "removes item when quantity is 0 or less", %{conn: conn, variant: variant} do
+      conn =
+        conn
+        |> conn_with_cart(variant.id, 3)
+        |> post(~p"/cart/update", %{"variant_id" => variant.id, "quantity" => "0"})
+
+      cart = get_session(conn, "cart")
+      assert cart == []
+    end
+  end
+end