Fix Beacon admin routing — mount platform pages without site prefix

Beacon admin pages (/beacon, /beacon/users, etc.) were being mounted
with /:site prefix like site-scoped pages, making them inaccessible.

Now the router separates:
- /beacon/* routes — mounted directly, no site prefix (platform-level)
- All other routes — mounted with /:site prefix (site-scoped)

PageLive mount handles pages without site param by using the first
running site for the beacon_page context.

Author: bcardarella

lib/beacon/live_admin/live/page_live.ex

@@ -30,6 +30,25 @@ defmodule Beacon.LiveAdmin.PageLive do
     assign_mount(socket, site, page, sites, pages, params)
   end
 
+  # Beacon admin pages (no site param — platform-level)
+  def mount(params, %{"pages" => pages} = session, socket) when not is_map_key(params, "site") do
+    sites = Beacon.LiveAdmin.Cluster.running_sites()
+    # Use the first running site for platform-level pages
+    site = List.first(sites) || :beacon
+
+    current_url =
+      Map.get(session, "beacon_live_admin_page_url") ||
+        raise Beacon.LiveAdmin.PageNotFoundError, """
+        failed to resolve Beacon.LiveAdmin page URL
+
+        You must add Beacon.LiveAdmin.Plug to the :browser pipeline that beacon_live_admin is piped through.
+        """
+
+    page = lookup_page!(socket, current_url)
+
+    assign_mount(socket, site, page, sites, pages, params)
+  end
+
   def mount(_params, _session, _socket) do
     raise Beacon.LiveAdmin.PageNotFoundError, """
     failed to resolve Beacon.LiveAdmin page URL

lib/beacon/live_admin/router.ex

@@ -106,7 +106,16 @@ defmodule Beacon.LiveAdmin.Router do
         live_session session_name, session_opts do
           live "/", Beacon.LiveAdmin.HomeLive, :index, as: :beacon_live_admin_home
 
-          for {path, page_module, live_action, _session} = page <- pages do
+          # Beacon admin pages — platform-level, no site prefix
+          for {path, page_module, live_action, _session} = page <- pages,
+              String.starts_with?(path, "/beacon") do
+            route_opts = Beacon.LiveAdmin.Router.__route_options__(opts, page)
+            live path, Beacon.LiveAdmin.PageLive, live_action, route_opts
+          end
+
+          # Site-scoped pages — prefixed with /:site
+          for {path, page_module, live_action, _session} = page <- pages,
+              not String.starts_with?(path, "/beacon") do
             route_opts = Beacon.LiveAdmin.Router.__route_options__(opts, page)
             path = "/:site#{path}"
             live path, Beacon.LiveAdmin.PageLive, live_action, route_opts