warning if JavaScript was not built

lib/Controller/PageController.php

@@ -30,9 +30,10 @@ class PageController extends Controller {
 	 * @return TemplateResponse
 	 */
 	public function index() {
+		$devMode = !is_file(dirname(__FILE__).'/../../js/notes.js');
 		$response = new TemplateResponse(
 			$this->appName,
-			'main',
+			$devMode ? 'dev-mode' : 'main',
 			[ ]
 		);
 

templates/dev-mode.php

@@ -0,0 +1,4 @@
+<div id="app-content" style="margin: 2em 3em;">
+	<h2><?php echo $l->t('Development Mode'); ?></h2>
+	<p><?php echo $l->t('It looks that the notes app was installed from a development source. Please install it from the official <a href="%1$s">Nextcloud App Store</a> instead. If you want to build the Notes app by yourself, please follow the <a href="%2$s">developer instructions</a>.', [ 'https://apps.nextcloud.com/apps/notes', 'https://github.com/nextcloud/notes#warning-developer-info' ]); ?></p>
+</div>