update dark mode handling

frontend/lib/provider/theme_provider.dart

@@ -0,0 +1,34 @@
+
+import 'package:flutter/material.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+Future<void> saveDarkMode(bool isDark) async {
+  final prefs = await SharedPreferences.getInstance();
+  await prefs.setBool('darkMode', isDark);
+}
+
+Future<bool> loadDarkMode() async {
+  final prefs = await SharedPreferences.getInstance();
+  return prefs.getBool('darkMode') ?? false; // default light
+}
+
+class ThemeProvider extends ChangeNotifier {
+  bool _isDark = false;
+
+  bool get isDark => _isDark;
+
+  ThemeProvider() {
+    _load(); // load saved value on startup
+  }
+
+  void _load() async {
+    _isDark = await loadDarkMode();
+    notifyListeners();
+  }
+
+  void setDarkMode(bool value) async {
+    _isDark = value;
+    notifyListeners();
+    await saveDarkMode(value);
+  }
+}