fix: code-review findings — error handling, null safety, persist settings, auth header

lib/screens/cloud_screen.dart

@@ -43,7 +43,10 @@ class _CloudScreenState extends State<CloudScreen> {
   Future<void> _verbindeCloud() async {
     final user = AuthService().benutzer;
     if (user.isNotEmpty) {
-      await widget.cloud.login(user);
+      final ok = await widget.cloud.login(user);
+      if (!ok && mounted) {
+        setState(() => _setzeStatus('Cloud-Login fehlgeschlagen', ok: false));
+      }
     }
   }
 
@@ -91,6 +94,7 @@ class _CloudScreenState extends State<CloudScreen> {
   Future<void> _upload() async {
     setState(() => _ladt = true);
     _setzeStatus('Suche lokale Songs...');
+    try {
     final dir = Directory('${(await getApplicationDocumentsDirectory()).path}/music');
     if (!await dir.exists()) {
       setState(() { _ladt = false; _setzeStatus('Keine lokalen Songs', ok: false); });
@@ -110,11 +114,16 @@ class _CloudScreenState extends State<CloudScreen> {
       _setzeStatus('$count Songs hochgeladen', ok: count > 0);
       MeloLogger().aktion('cloud_upload', {'count': count});
     }
+    } catch (e) {
+      MeloLogger().fehler('cloud_upload_path', e);
+      if (mounted) setState(() { _ladt = false; _setzeStatus('Fehler beim Upload', ok: false); });
+    }
   }
 
   Future<void> _download() async {
-    setState(() => _ladt = true);
+    setState(() { _ladt = true; });
     _setzeStatus('Vergleiche mit Server...');
+    try {
     final dir = Directory('${(await getApplicationDocumentsDirectory()).path}/music');
     if (!await dir.exists()) await dir.create(recursive: true);
     final localFiles = dir.listSync().whereType<File>()
@@ -122,7 +131,7 @@ class _CloudScreenState extends State<CloudScreen> {
     final serverSongs = await widget.cloud.listSongs();
     int downloaded = 0;
     for (final song in serverSongs) {
-      final title = song['title'].toString();
+      final title = (song['title'] ?? 'unknown').toString();
       if (localFiles.contains(title)) continue;
       _setzeStatus('Download: $title...');
       final sid = song['id'].toString();
@@ -135,6 +144,10 @@ class _CloudScreenState extends State<CloudScreen> {
       _setzeStatus('$downloaded Songs heruntergeladen', ok: true);
       MeloLogger().aktion('cloud_download', {'count': downloaded});
     }
+    } catch (e) {
+      MeloLogger().fehler('cloud_download_path', e);
+      if (mounted) setState(() { _ladt = false; _setzeStatus('Fehler beim Download', ok: false); });
+    }
   }
 
   void _setzeStatus(String msg, {bool ok = false}) {