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>

test/berrypod/analytics/retention_worker_test.exs

@@ -4,6 +4,11 @@ defmodule Berrypod.Analytics.RetentionWorkerTest do
   alias Berrypod.Analytics.{Event, RetentionWorker}
   alias Berrypod.Repo
 
+  setup do
+    Repo.delete_all(Event)
+    :ok
+  end
+
   test "deletes events older than 12 months" do
     old = DateTime.add(DateTime.utc_now(), -400, :day) |> DateTime.truncate(:second)
     recent = DateTime.utc_now() |> DateTime.truncate(:second)