fix coming soon page: crash, logo centering, admin login link

Add AnalyticsHook to the coming_soon live session — the shop layout
fires an analytics:screen JS event but the session had no handler,
crashing the page on connect.

Centre the logo image (display:block from CSS reset needs
margin-inline:auto). Add a subtle "Admin" link at the bottom using
flex flow rather than fixed/absolute positioning so it works in
iframes.

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

lib/berrypod_web/router.ex

@@ -243,7 +243,8 @@ defmodule BerrypodWeb.Router do
       layout: {BerrypodWeb.Layouts, :shop},
       on_mount: [
         {BerrypodWeb.SetupHook, :require_admin},
-        {BerrypodWeb.ThemeHook, :mount_theme}
+        {BerrypodWeb.ThemeHook, :mount_theme},
+        {BerrypodWeb.AnalyticsHook, :track}
       ] do
       live "/coming-soon", Shop.ComingSoon, :index
     end