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/router.ex

@@ -81,6 +81,7 @@ defmodule BerrypodWeb.Router do
       live "/collections/:slug", Shop.Collection, :show
       live "/products/:id", Shop.ProductShow, :show
       live "/cart", Shop.Cart, :index
+      live "/search", Shop.Search, :index
       live "/checkout/success", Shop.CheckoutSuccess, :show
       live "/orders", Shop.Orders, :index
       live "/orders/:order_number", Shop.OrderDetail, :show
@@ -88,6 +89,11 @@ defmodule BerrypodWeb.Router do
 
     # Checkout (POST — creates Stripe session and redirects)
     post "/checkout", CheckoutController, :create
+
+    # Cart form actions (no-JS fallbacks for LiveView cart events)
+    post "/cart/add", CartController, :add
+    post "/cart/remove", CartController, :remove
+    post "/cart/update", CartController, :update_item
   end
 
   # Health check (no auth, no theme loading — for load balancers and uptime monitors)