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/cart.ex

@@ -213,11 +213,18 @@ defmodule BerrypodWeb.ShopComponents.Cart do
 
         <div class="cart-item-actions">
           <%= if @show_quantity_controls do %>
-            <div class="cart-qty-group">
+            <form
+              action="/cart/update"
+              method="post"
+              phx-submit="update_quantity_form"
+              class="cart-qty-group"
+            >
+              <input type="hidden" name="_csrf_token" value={Phoenix.Controller.get_csrf_token()} />
+              <input type="hidden" name="variant_id" value={@item.variant_id} />
               <button
-                type="button"
-                phx-click="decrement"
-                phx-value-id={@item.variant_id}
+                type="submit"
+                name="quantity"
+                value={@item.quantity - 1}
                 class="cart-qty-btn"
                 aria-label={"Decrease quantity of #{@item.name}"}
               >
@@ -227,15 +234,15 @@ defmodule BerrypodWeb.ShopComponents.Cart do
                 {@item.quantity}
               </span>
               <button
-                type="button"
-                phx-click="increment"
-                phx-value-id={@item.variant_id}
+                type="submit"
+                name="quantity"
+                value={@item.quantity + 1}
                 class="cart-qty-btn"
                 aria-label={"Increase quantity of #{@item.name}"}
               >
                 +
               </button>
-            </div>
+            </form>
           <% else %>
             <span class="cart-qty-text">
               Qty: {@item.quantity}
@@ -306,15 +313,17 @@ defmodule BerrypodWeb.ShopComponents.Cart do
 
   def cart_remove_button(assigns) do
     ~H"""
-    <button
-      type="button"
-      phx-click="remove_item"
-      phx-value-id={@variant_id}
-      class="cart-remove-btn"
-      aria-label={"Remove #{@item_name} from cart"}
-    >
-      Remove
-    </button>
+    <form action="/cart/remove" method="post" phx-submit="remove_item_form" class="cart-remove-form">
+      <input type="hidden" name="_csrf_token" value={Phoenix.Controller.get_csrf_token()} />
+      <input type="hidden" name="variant_id" value={@variant_id} />
+      <button
+        type="submit"
+        class="cart-remove-btn"
+        aria-label={"Remove #{@item_name} from cart"}
+      >
+        Remove
+      </button>
+    </form>
     """
   end