updated project linter-rules and enforced them

lib/widget/fileViewer.dart

@@ -37,7 +37,7 @@ class _FileViewerState extends State<FileViewer> {
   Widget build(BuildContext context) {
     AppBar appbar({List actions = const []}) {
       return AppBar(
-        title: Text(widget.path.split("/").last),
+        title: Text(widget.path.split('/').last),
         actions: [
           IconButton(
               onPressed: () => Navigator.of(context).push(
@@ -59,12 +59,12 @@ class _FileViewerState extends State<FileViewer> {
       );
     }
 
-    switch(openExternal ? "" : widget.path.split(".").last.toLowerCase()) {
-      case "png":
-      case "jpg":
-      case "jpeg":
-      case "webp":
-      case "gif":
+    switch(openExternal ? '' : widget.path.split('.').last.toLowerCase()) {
+      case 'png':
+      case 'jpg':
+      case 'jpeg':
+      case 'webp':
+      case 'gif':
         return Scaffold(
             appBar: appbar(
                 actions: [
@@ -84,7 +84,7 @@ class _FileViewerState extends State<FileViewer> {
         );
 
 
-      case "pdf":
+      case 'pdf':
         return Scaffold(
           appBar: appbar(),
           body: SfPdfViewer.file(
@@ -105,11 +105,11 @@ class _FileViewerState extends State<FileViewer> {
         });
 
         return PlaceholderView(
-          text: "Datei extern geöffnet",
+          text: 'Datei extern geöffnet',
           icon: Icons.open_in_new,
           button: TextButton(
             onPressed: () => Navigator.of(context).pop(),
-            child: const Text("Zurück"),
+            child: const Text('Zurück'),
           ),
         );
     }