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/shop_components/layout.ex

@@ -755,10 +755,10 @@ defmodule BerrypodWeb.ShopComponents.Layout do
             />
           </svg>
         </.link>
-        <button
-          type="button"
-          class="header-icon-btn"
+        <a
+          href="/search"
           phx-click={Phoenix.LiveView.JS.dispatch("open-search", to: "#search-modal")}
+          class="header-icon-btn"
           aria-label="Search"
         >
           <svg
@@ -772,11 +772,11 @@ defmodule BerrypodWeb.ShopComponents.Layout do
             <circle cx="11" cy="11" r="8"></circle>
             <path d="M21 21l-4.35-4.35"></path>
           </svg>
-        </button>
-        <button
-          type="button"
-          class="header-icon-btn"
+        </a>
+        <a
+          href="/cart"
           phx-click={open_cart_drawer_js()}
+          class="header-icon-btn"
           aria-label="Cart"
         >
           <svg
@@ -797,7 +797,7 @@ defmodule BerrypodWeb.ShopComponents.Layout do
             </span>
           <% end %>
           <span class="sr-only">Cart ({@cart_count})</span>
-        </button>
+        </a>
       </div>
     </header>
     """