frontend: declare the empty toplevel export the codegen reads

routes.gen.js spreads `.toplevel` for every plugin uniformly, but only a
few own full-screen routes, so Rollup warned on eleven of them every
build. The `|| []` guard was always doing its job - the warning was noise,
and noise in a build log is where a real warning goes to hide.

plugins/notifications/frontend/routes.js

@@ -39,3 +39,8 @@ export default [
     meta: { plugin: 'notifications' }
   }
 ]
+
+// No full-screen routes: every view in this plugin renders inside
+// AppLayout. Declared so routes.gen.js can read `.toplevel` off the module
+// without Rollup warning about a missing export.
+export const toplevel = []