Admin analytics: heatmap per device, anteprima staff e analisi costi.

Separa heatmap mobile/desktop, opt-out analytics per operatori, dashboard costi con KPI e trend mensili.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/views/layouts/admin.html.erb

@@ -6,6 +6,7 @@
   <meta name="robots" content="noindex, nofollow">
   <%= csrf_meta_tags %>
   <link rel="stylesheet" href="/admin.css?v=15">
+  <%= yield :head %>
   <% if content_for?(:replay_archive_styles) %>
     <link rel="stylesheet" href="/marketing.css?v=42">
   <% end %>
@@ -32,6 +33,7 @@
         <%= link_to t("admin.layout.nav.youtube"), admin_youtube_platform_path, class: ("active" if controller_name == "youtube") %>
         <%= link_to t("admin.layout.nav.sessions"), admin_sessions_path, class: ("active" if controller_name == "sessions") %>
         <%= link_to t("admin.layout.nav.analytics"), admin_analytics_path, class: ("active" if controller_name == "analytics") %>
+        <%= link_to t("admin.layout.nav.costs"), admin_costs_path, class: ("active" if controller_name.in?(%w[costs cost_entries])) %>
         <%= link_to t("admin.layout.nav.stream_nodes"), admin_stream_nodes_path, class: ("active" if controller_name == "stream_nodes") %>
         <%= link_to t("admin.layout.nav.password"), edit_admin_password_path %>
         <%= button_to t("admin.layout.nav.logout"), admin_logout_path, method: :delete %>