show proper 404 page for missing custom pages instead of redirecting

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

lib/berrypod_web/live/shop/custom_page.ex

@@ -15,23 +15,13 @@ defmodule BerrypodWeb.Shop.CustomPage do
     cond do
       is_nil(page) ->
         record_broken_url("/#{slug}")
-
-        {:noreply,
-         socket
-         |> put_flash(:error, "Page not found")
-         |> push_navigate(to: ~p"/")}
+        raise BerrypodWeb.NotFoundError
 
       page.type != "custom" ->
-        {:noreply,
-         socket
-         |> put_flash(:error, "Page not found")
-         |> push_navigate(to: ~p"/")}
+        raise BerrypodWeb.NotFoundError
 
       page.published != true and not socket.assigns.is_admin ->
-        {:noreply,
-         socket
-         |> put_flash(:error, "Page not found")
-         |> push_navigate(to: ~p"/")}
+        raise BerrypodWeb.NotFoundError
 
       true ->
         extra = Pages.load_block_data(page.blocks, socket.assigns)

lib/berrypod_web/not_found_error.ex

@@ -0,0 +1,10 @@
+defmodule BerrypodWeb.NotFoundError do
+  @moduledoc """
+  Raised in LiveViews when a resource can't be found.
+
+  Implements `Plug.Exception` so Phoenix renders the 404 error page
+  instead of a 500.
+  """
+
+  defexception message: "not found", plug_status: 404
+end