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

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),
             ),