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}) {

lib/screens/home_screen.dart

@@ -9,7 +9,6 @@ import '../widgets/melo_header.dart';
 import '../widgets/statistik_card.dart';
 import '../widgets/tag_leiste.dart';
 import '../widgets/song_tile.dart';
-import '../widgets/navidrome_browser.dart';
 import '../widgets/playlist_sheet.dart';
 import 'download_screen.dart';
 import 'cloud_screen.dart';

lib/screens/settings_screen.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 import '../config/app_config.dart';
 import '../utils/farb_theme.dart';
 import 'cloud_screen.dart';
@@ -15,6 +16,17 @@ class SettingsScreen extends StatefulWidget {
 class _SettingsScreenState extends State<SettingsScreen> {
   bool _diagnose = AppConfig.sendeDiagnosedaten;
 
+  @override
+  void initState() {
+    super.initState();
+    _ladeDiagnose();
+  }
+
+  Future<void> _ladeDiagnose() async {
+    final p = await SharedPreferences.getInstance();
+    if (mounted) setState(() => _diagnose = p.getBool('diagnose_daten') ?? AppConfig.sendeDiagnosedaten);
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(
@@ -84,9 +96,11 @@ class _SettingsScreenState extends State<SettingsScreen> {
               ),
               value: _diagnose,
               activeThumbColor: MeloTheme.rot,
-              onChanged: (v) {
+              onChanged: (v) async {
                 setState(() => _diagnose = v);
                 AppConfig.sendeDiagnosedaten = v;
+                final p = await SharedPreferences.getInstance();
+                p.setBool('diagnose_daten', v);
               },
               secondary: const Icon(Icons.bug_report_outlined, color: MeloTheme.rot, size: 22),
             ),

lib/services/auth_service.dart

@@ -21,10 +21,15 @@ class AuthService {
   String? get token => _token;
 
   /// Auth-Header für API-Requests
-  Map<String, String> get authHeader => {
-    'Authorization': 'Bearer ${_token ?? ''}',
-    'Content-Type': 'application/json',
-  };
+  Map<String, String> get authHeader {
+    final headers = <String, String>{
+      'Content-Type': 'application/json',
+    };
+    if (_token != null && _token!.isNotEmpty) {
+      headers['Authorization'] = 'Bearer $_token';
+    }
+    return headers;
+  }
 
   /// Lädt gespeicherten Token beim App-Start
   Future<void> initialisieren() async {