Catch uncaught async errors and surface response-shape failures

Wrap main in runZonedGuarded and install FlutterError.onError /
PlatformDispatcher.onError so background async failures get logged
instead of dying silently. Guard PrefUtil.init so a SharedPreferences
platform error no longer aborts startup. Recognize TypeError and
FormatException in dioErrorToUserMessage to give a clearer message
when the backend returns an unexpected JSON shape or bad timestamp.
Wrap Settings._save in try/catch so prefs write failures show a
snackbar instead of leaving the Save button silently dead.

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

frontend/lib/main.dart

@@ -1,3 +1,6 @@
+import 'dart:async';
+
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:frontend/theme/catppuccin_mocha_theme.dart';
 import 'package:frontend/utils/pref_utils.dart';
@@ -5,10 +8,32 @@ import 'package:provider/provider.dart';
 import 'navigation.dart';
 import 'theme/gruvbox_theme.dart';
 
-void main() async {
+void main() {
+  runZonedGuarded(
+    () async {
       WidgetsFlutterBinding.ensureInitialized();
+
+      FlutterError.onError = (details) {
+        FlutterError.presentError(details);
+        debugPrint('FlutterError: ${details.exceptionAsString()}');
+      };
+      PlatformDispatcher.instance.onError = (error, stack) {
+        debugPrint('Uncaught platform error: $error\n$stack');
+        return true;
+      };
+
+      try {
         await PrefUtil.init();
+      } catch (e, stack) {
+        debugPrint('PrefUtil.init failed, continuing with defaults: $e\n$stack');
+      }
+
       runApp(const MainApp());
+    },
+    (error, stack) {
+      debugPrint('Uncaught zone error: $error\n$stack');
+    },
+  );
 }
 
 final class MainApp extends StatelessWidget {

frontend/lib/settings/settings.dart

@@ -69,7 +69,11 @@ class _SettingsState extends State<Settings> {
 
   Future<void> _save() async {
     if (!_formKey.currentState!.validate()) return;
-    final urlOk = await PrefUtil.setValue('base_url', _baseUrlController.text);
+    try {
+      final urlOk = await PrefUtil.setValue(
+        'base_url',
+        _baseUrlController.text,
+      );
       if (!mounted) return;
       final keyOk = await PrefUtil.setValue(
         'api_key',
@@ -84,6 +88,10 @@ class _SettingsState extends State<Settings> {
       } else {
         UISnackbars.showError(context, 'Failed to save settings');
       }
+    } catch (e) {
+      if (!mounted) return;
+      UISnackbars.showError(context, 'Failed to save settings: $e');
+    }
   }
 
   @override

frontend/lib/utils/oott_api.dart

@@ -59,6 +59,10 @@ LogInterceptor _buildLogInterceptor() => LogInterceptor(
 );
 
 String dioErrorToUserMessage(Object error) {
+  if (error is TypeError || error is FormatException) {
+    debugPrint('Backend response shape error: $error');
+    return 'Unexpected response shape from backend.';
+  }
   if (error is! DioException) {
     debugPrint('Non-Dio error from backend call: $error');
     return 'Unexpected error.';