다크 모드 전역 상태와 테마 기반 추가

userfront/lib/core/theme/theme_controller.dart

@@ -0,0 +1,32 @@
+import 'package:flutter/material.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+class ThemeController extends ValueNotifier<ThemeMode> {
+  ThemeController._() : super(ThemeMode.light);
+
+  static const storageKey = 'userfront_theme';
+  static final ThemeController instance = ThemeController._();
+
+  bool get isDark => value == ThemeMode.dark;
+
+  Future<void> restore() async {
+    final prefs = await SharedPreferences.getInstance();
+    final stored = prefs.getString(storageKey);
+    value = stored == 'dark' ? ThemeMode.dark : ThemeMode.light;
+  }
+
+  Future<void> setThemeMode(ThemeMode mode) async {
+    if (value != mode) {
+      value = mode;
+    }
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(
+      storageKey,
+      mode == ThemeMode.dark ? 'dark' : 'light',
+    );
+  }
+
+  Future<void> toggle() {
+    return setThemeMode(isDark ? ThemeMode.light : ThemeMode.dark);
+  }
+}