show proper 404 page for missing custom pages instead of redirecting

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

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