extract setup wizard to dedicated /admin/setup page

Move the setup stepper out of the dashboard into its own LiveView.
Dashboard now redirects to setup when site isn't live, and shows
stats-only view once live. Also cleans up button component variant
handling, fixes alert CSS, and removes stale demo.html.

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

lib/berrypod_web/components/layouts/admin.html.heex

@@ -43,6 +43,14 @@
       <%!-- nav links --%>
       <nav class="flex-1 p-2" aria-label="Admin navigation">
         <ul class="admin-nav">
+          <li :if={!@site_live}>
+            <.link
+              navigate={~p"/admin/setup"}
+              class={admin_nav_active?(@current_path, "/admin/setup")}
+            >
+              <.icon name="hero-rocket-launch" class="size-5" /> Setup
+            </.link>
+          </li>
           <li>
             <.link
               navigate={~p"/admin"}