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/live/shop/collection.ex

@@ -176,13 +176,16 @@ defmodule BerrypodWeb.Shop.Collection do
         </ul>
       </nav>
 
-      <form phx-change="sort_changed">
+      <form action={~p"/collections/#{@current_slug || "all"}"} method="get" phx-change="sort_changed">
         <.shop_select
           name="sort"
           options={@sort_options}
           selected={@current_sort}
           aria-label="Sort products"
         />
+        <noscript>
+          <button type="submit" class="themed-button collection-sort-submit">Sort</button>
+        </noscript>
       </form>
     </div>
     """

lib/berrypod_web/live/shop/search.ex

@@ -0,0 +1,75 @@
+defmodule BerrypodWeb.Shop.Search do
+  use BerrypodWeb, :live_view
+
+  alias Berrypod.Search
+
+  @impl true
+  def mount(_params, _session, socket) do
+    {:ok, assign(socket, :page_title, "Search")}
+  end
+
+  @impl true
+  def handle_params(params, _uri, socket) do
+    query = params["q"] || ""
+    results = if query != "", do: Search.search(query), else: []
+
+    {:noreply,
+     socket
+     |> assign(:search_page_query, query)
+     |> assign(:search_page_results, results)}
+  end
+
+  @impl true
+  def handle_event("search_submit", %{"q" => query}, socket) do
+    {:noreply, push_patch(socket, to: ~p"/search?q=#{query}")}
+  end
+
+  @impl true
+  def render(assigns) do
+    ~H"""
+    <.shop_layout {layout_assigns(assigns)} active_page="search">
+      <main id="main-content" class="page-container">
+        <.page_title text="Search" />
+
+        <form action="/search" method="get" phx-submit="search_submit" class="search-page-form">
+          <input
+            type="search"
+            name="q"
+            value={@search_page_query}
+            placeholder="Search products..."
+            class="themed-input"
+          />
+          <button type="submit" class="themed-button">Search</button>
+        </form>
+
+        <%= if @search_page_results != [] do %>
+          <p class="search-page-count">
+            {length(@search_page_results)} {if length(@search_page_results) == 1,
+              do: "result",
+              else: "results"} for "{@search_page_query}"
+          </p>
+          <.product_grid theme_settings={@theme_settings}>
+            <%= for product <- @search_page_results do %>
+              <.product_card
+                product={product}
+                theme_settings={@theme_settings}
+                mode={@mode}
+                variant={:default}
+              />
+            <% end %>
+          </.product_grid>
+        <% else %>
+          <%= if @search_page_query != "" do %>
+            <div class="collection-empty">
+              <p>No products found for "{@search_page_query}"</p>
+              <.link navigate="/collections/all" class="collection-empty-link">
+                Browse all products
+              </.link>
+            </div>
+          <% end %>
+        <% end %>
+      </main>
+    </.shop_layout>
+    """
+  end
+end