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/components/page_templates/pdp.html.heex

@@ -25,26 +25,36 @@
       <div>
         <.product_info product={@product} display_price={@display_price} />
 
-        <%!-- Dynamic variant selectors --%>
-        <%= for option_type <- @option_types do %>
-          <.variant_selector
-            option_type={option_type}
-            selected={@selected_options[option_type.name]}
-            available={@available_options[option_type.name] || []}
-            mode={@mode}
+        <form action="/cart/add" method="post" phx-submit="add_to_cart">
+          <input type="hidden" name="_csrf_token" value={Phoenix.Controller.get_csrf_token()} />
+          <input
+            type="hidden"
+            name="variant_id"
+            value={@selected_variant && @selected_variant.id}
           />
-        <% end %>
+          <input type="hidden" name="quantity" value={@quantity} />
 
-        <%!-- Fallback for products with no variant options --%>
-        <div
-          :if={@option_types == []}
-          class="pdp-variant-fallback"
-        >
-          One size
-        </div>
+          <%!-- Dynamic variant selectors --%>
+          <%= for option_type <- @option_types do %>
+            <.variant_selector
+              option_type={option_type}
+              selected={@selected_options[option_type.name]}
+              available={@available_options[option_type.name] || []}
+              mode={@mode}
+            />
+          <% end %>
 
-        <.quantity_selector quantity={@quantity} in_stock={@product.in_stock} />
-        <.add_to_cart_button mode={@mode} />
+          <%!-- Fallback for products with no variant options --%>
+          <div
+            :if={@option_types == []}
+            class="pdp-variant-fallback"
+          >
+            One size
+          </div>
+
+          <.quantity_selector quantity={@quantity} in_stock={@product.in_stock} />
+          <.add_to_cart_button mode={@mode} />
+        </form>
         <.trust_badges :if={@theme_settings.pdp_trust_badges} />
         <.product_details product={@product} />
       </div>