include browser/os/screen_size in e-commerce analytics events

Event call sites (product_view, add_to_cart, checkout_start, purchase)
were only passing visitor_hash and pathname, leaving browser, OS, screen
size and country nil. Add AnalyticsHook.attrs/1 helper to extract common
analytics fields from socket assigns, and use it in all LiveView event
call sites. Checkout controller reads the same fields from the session.

Also fix plug analytics test to clear stale events before assertions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/berrypod_web/plugs/analytics_test.exs

@@ -9,6 +9,7 @@ defmodule BerrypodWeb.Plugs.AnalyticsTest do
   setup do
     send(Buffer, :flush)
     :timer.sleep(50)
+    Repo.delete_all(Event)
     :ok
   end