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/live/shop/coming_soon.ex

@@ -19,6 +19,7 @@ defmodule BerrypodWeb.Shop.ComingSoon do
           We're getting things ready. Check back soon.
         </p>
       </div>
+      <a href="/admin" class="coming-soon-admin-link">Admin</a>
     </main>
     """
   end