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lib/providers/theme_provider.dart

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+/// [ChangeNotifier] that owns [ThemeMode] and persists it locally.
+library;
+
+import 'package:flutter/material.dart';
+
+import '../services/storage_service.dart';
+import '../utils/helpers.dart';
+
+/// Provides [themeMode] to [MaterialApp] and writes changes to storage.
+class ThemeController extends ChangeNotifier {
+  ThemeController(this._storage) {
+    _themeMode = themeModeFromName(_storage.themeModeName);
+  }
+
+  final StorageService _storage;
+  late ThemeMode _themeMode;
+
+  ThemeMode get themeMode => _themeMode;
+
+  bool get isDark => _themeMode == ThemeMode.dark;
+
+  Future<void> setThemeMode(ThemeMode mode) async {
+    if (mode == _themeMode) return;
+    _themeMode = mode;
+    notifyListeners();
+    await _storage.setThemeModeName(nameFromThemeMode(mode));
+  }
+
+  Future<void> cycleThemeMode() {
+    final next = switch (_themeMode) {
+      ThemeMode.system => ThemeMode.light,
+      ThemeMode.light => ThemeMode.dark,
+      ThemeMode.dark => ThemeMode.system,
+    };
+    return setThemeMode(next);
+  }
+}