Surface persistence and fetch errors instead of swallowing them

PrefUtil.setValue now returns the underlying SharedPreferences result
and throws on unsupported types; settings save reports failures rather
than always showing success. Device detail and event history report
the real Dio error message and skip cancellations.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/lib/devices/device_detail.dart

@@ -1,3 +1,4 @@
+import 'package:dio/dio.dart';
 import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 
@@ -42,8 +43,9 @@ class _DeviceDetailState extends State<DeviceDetail> {
       });
     } catch (e) {
       if (!mounted) return;
+      if (e is DioException && e.type == DioExceptionType.cancel) return;
       setState(() {
-        _error = e.toString();
+        _error = dioErrorToUserMessage(e);
         _isLoading = false;
       });
     }
@@ -140,7 +142,10 @@ class _DeviceHeader extends StatelessWidget {
           child: Column(
             crossAxisAlignment: CrossAxisAlignment.start,
             children: [
-              SelectableText(device.ipv4Address, style: theme.textTheme.headlineSmall),
+              SelectableText(
+                device.ipv4Address,
+                style: theme.textTheme.headlineSmall,
+              ),
               const SizedBox(height: 4),
               if (device.isRegistered)
                 StatusBadge(label: 'Registered', color: BadgeColor.success)
@@ -166,7 +171,10 @@ class _DeviceInfoCard extends StatelessWidget {
   Widget build(BuildContext context) {
     final formatter = FriendlyDateFormatter();
     final rows = <(String, String)>[
-      ('Name', device.name == null || device.name!.isEmpty ? '—' : device.name!),
+      (
+        'Name',
+        device.name == null || device.name!.isEmpty ? '—' : device.name!,
+      ),
       ('MAC Address', device.macAddress),
       ('IP Address', device.ipv4Address),
       ('Vendor', device.vendor.isEmpty ? '—' : device.vendor),
@@ -251,7 +259,9 @@ class _InfoRow extends StatelessWidget {
               ),
             ),
           ),
-          Expanded(child: SelectableText(value, style: theme.textTheme.bodyMedium)),
+          Expanded(
+            child: SelectableText(value, style: theme.textTheme.bodyMedium),
+          ),
         ],
       ),
     );

frontend/lib/devices/device_event_history.dart

@@ -1,3 +1,4 @@
+import 'package:dio/dio.dart';
 import 'package:fl_chart/fl_chart.dart';
 import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
@@ -98,8 +99,9 @@ class _DeviceEventHistoryState extends State<DeviceEventHistory> {
       });
     } catch (e) {
       if (!mounted) return;
+      if (e is DioException && e.type == DioExceptionType.cancel) return;
       setState(() {
-        _error = e.toString();
+        _error = dioErrorToUserMessage(e);
         _isLoading = false;
       });
     }
@@ -115,9 +117,9 @@ class _DeviceEventHistoryState extends State<DeviceEventHistory> {
     }
 
     if (_error != null) {
-      return const Padding(
-        padding: EdgeInsets.symmetric(vertical: 16),
-        child: Center(child: Text('Failed to load event history')),
+      return Padding(
+        padding: const EdgeInsets.symmetric(vertical: 16),
+        child: Center(child: Text('Failed to load event history: $_error')),
       );
     }
 

frontend/lib/main.dart

@@ -44,10 +44,11 @@ class AppState extends ChangeNotifier {
   String get themeKey => _themeKey;
   ThemeData get theme => _themes[_themeKey]!;
 
-  void setTheme(String key) {
-    if (!_themes.containsKey(key) || key == _themeKey) return;
+  Future<bool> setTheme(String key) async {
+    if (!_themes.containsKey(key)) return false;
+    if (key == _themeKey) return true;
     _themeKey = key;
-    PrefUtil.setValue('theme', key);
     notifyListeners();
+    return PrefUtil.setValue('theme', key);
   }
 }

frontend/lib/settings/settings.dart

@@ -67,13 +67,23 @@ class _SettingsState extends State<Settings> {
     }
   }
 
-  void _save() {
+  Future<void> _save() async {
     if (!_formKey.currentState!.validate()) return;
-    PrefUtil.setValue('base_url', _baseUrlController.text);
-    PrefUtil.setValue('api_key', XOR().xorEncode(_apiKeyController.text));
-    BackendAPI.instance.reconfigureFromPrefs();
-    context.read<AppState>().setTheme(_selectedTheme);
-    UISnackbars.showSuccess(context, 'Settings saved successfully');
+    final urlOk = await PrefUtil.setValue('base_url', _baseUrlController.text);
+    if (!mounted) return;
+    final keyOk = await PrefUtil.setValue(
+      'api_key',
+      XOR().xorEncode(_apiKeyController.text),
+    );
+    if (!mounted) return;
+    final themeOk = await context.read<AppState>().setTheme(_selectedTheme);
+    if (!mounted) return;
+    if (urlOk && keyOk && themeOk) {
+      BackendAPI.instance.reconfigureFromPrefs();
+      UISnackbars.showSuccess(context, 'Settings saved successfully');
+    } else {
+      UISnackbars.showError(context, 'Failed to save settings');
+    }
   }
 
   @override

frontend/lib/utils/pref_utils.dart

@@ -10,15 +10,18 @@ class PrefUtil {
     return preferences;
   }
 
-  static void setValue(String key, Object value) {
+  static Future<bool> setValue(String key, Object value) {
     switch (value) {
       case String s:
-        preferences.setString(key, s);
+        return preferences.setString(key, s);
       case bool b:
-        preferences.setBool(key, b);
+        return preferences.setBool(key, b);
       case int i:
-        preferences.setInt(key, i);
+        return preferences.setInt(key, i);
       default:
+        throw ArgumentError(
+          'Unsupported pref value type: ${value.runtimeType}',
+        );
     }
   }