add admin dashboard with setup checklist and stats

Dashboard at /admin shows setup progress (when not live), stat cards
(orders, revenue, products), and recent paid orders table. Replaces
the old AdminController redirect. Add Dashboard to sidebar nav as
first item, update admin bar and theme editor links to /admin.

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

lib/simpleshop_theme_web/live/admin/theme/index.html.heex

@@ -36,7 +36,7 @@
         </div>
       <% else %>
         <.link
-          href={~p"/admin/orders"}
+          href={~p"/admin"}
           class="inline-flex items-center gap-1 text-sm text-base-content/60 hover:text-base-content mb-4"
         >
           <.icon name="hero-arrow-left-mini" class="size-4" /> Admin