feat(service): add demo data initializer service

- Implement DemoDataInitializerService to ensure essential user-specific data exists in demo environment
- Add methods to initialize UserAppSettings, UserContentPreferences, and User object in memory
- Include error handling and logging for data initialization process
- Service is designed specifically for demo environment, as production/development use backend API for data initialization

Author: fulleni

lib/app/services/demo_data_initializer_service.dart

@@ -0,0 +1,181 @@
+import 'package:core/core.dart';
+import 'package:data_repository/data_repository.dart';
+import 'package:flutter/material.dart'; // Required for ThemeMode, AppBaseTheme, etc.
+
+/// {@template demo_data_initializer_service}
+/// A service responsible for ensuring that essential user-specific data
+/// (like [UserAppSettings], [UserContentPreferences], and the [User] object
+/// itself) exists in the data in-memory clients when a user is first encountered
+/// in the demo environment.
+///
+/// This service is specifically designed for the in-memory data clients
+/// used in the demo environment. In production/development environments,
+/// the backend API is responsible for initializing user data.
+/// {@endtemplate}
+class DemoDataInitializerService {
+  /// {@macro demo_data_initializer_service}
+  const DemoDataInitializerService({
+    required DataRepository<UserAppSettings> userAppSettingsRepository,
+    required DataRepository<UserContentPreferences>
+        userContentPreferencesRepository,
+    required DataRepository<User> userRepository,
+  })  : _userAppSettingsRepository = userAppSettingsRepository,
+        _userContentPreferencesRepository = userContentPreferencesRepository,
+        _userRepository = userRepository;
+
+  final DataRepository<UserAppSettings> _userAppSettingsRepository;
+  final DataRepository<UserContentPreferences>
+      _userContentPreferencesRepository;
+  final DataRepository<User> _userRepository;
+
+  /// Initializes essential user-specific data in the in-memory clients
+  /// for the given [user].
+  ///
+  /// This method checks if [UserAppSettings], [UserContentPreferences],
+  /// and the [User] object itself exist for the provided user ID. If any
+  /// are missing, it creates them with default values.
+  ///
+  /// This prevents "READ FAILED" errors when the application attempts to
+  /// access these user-specific data points for a newly signed-in anonymous
+  /// user in the demo environment.
+  Future<void> initializeUserSpecificData(User user) async {
+    print(
+      '[DemoDataInitializerService] Initializing user-specific data for '
+      'user ID: ${user.id}',
+    );
+
+    // Ensure UserAppSettings exist
+    await _ensureUserAppSettingsExist(user.id);
+
+    // Ensure UserContentPreferences exist
+    await _ensureUserContentPreferencesExist(user.id);
+
+    // Ensure the User object itself is up-to-date in the user client
+    await _ensureUserClientUserExists(user);
+
+    print(
+      '[DemoDataInitializerService] User-specific data initialization '
+      'completed for user ID: ${user.id}',
+    );
+  }
+
+  /// Ensures that [UserAppSettings] exist for the given [userId].
+  /// If not found, creates default settings.
+  Future<void> _ensureUserAppSettingsExist(String userId) async {
+    try {
+      await _userAppSettingsRepository.read(id: userId, userId: userId);
+      print(
+        '[DemoDataInitializerService] UserAppSettings found for user ID: $userId.',
+      );
+    } on NotFoundException {
+      print(
+        '[DemoDataInitializerService] UserAppSettings not found for user ID: '
+        '$userId. Creating default settings.',
+      );
+      final defaultSettings = UserAppSettings(
+        id: userId,
+        displaySettings: const DisplaySettings(
+          baseTheme: AppBaseTheme.system,
+          accentTheme: AppAccentTheme.defaultBlue,
+          fontFamily: 'SystemDefault',
+          textScaleFactor: AppTextScaleFactor.medium,
+          fontWeight: AppFontWeight.regular,
+        ),
+        language: languagesFixturesData.firstWhere(
+          (l) => l.code == 'en',
+          orElse: () => throw StateError(
+            'Default language "en" not found in language fixtures.',
+          ),
+        ),
+        feedPreferences: const FeedDisplayPreferences(
+          headlineDensity: HeadlineDensity.standard,
+          headlineImageStyle: HeadlineImageStyle.smallThumbnail,
+          showSourceInHeadlineFeed: true,
+          showPublishDateInHeadlineFeed: true,
+        ),
+      );
+      await _userAppSettingsRepository.create(item: defaultSettings, userId: userId);
+      print(
+        '[DemoDataInitializerService] Default UserAppSettings created for '
+        'user ID: $userId.',
+      );
+    } catch (e, s) {
+      print(
+        '[DemoDataInitializerService] Error ensuring UserAppSettings exist '
+        'for user ID: $userId: $e\n$s',
+      );
+      rethrow;
+    }
+  }
+
+  /// Ensures that [UserContentPreferences] exist for the given [userId].
+  /// If not found, creates default preferences.
+  Future<void> _ensureUserContentPreferencesExist(String userId) async {
+    try {
+      await _userContentPreferencesRepository.read(id: userId, userId: userId);
+      print(
+        '[DemoDataInitializerService] UserContentPreferences found for user ID: $userId.',
+      );
+    } on NotFoundException {
+      print(
+        '[DemoDataInitializerService] UserContentPreferences not found for '
+        'user ID: $userId. Creating default preferences.',
+      );
+      final defaultPreferences = UserContentPreferences(
+        id: userId,
+        followedCountries: const [],
+        followedSources: const [],
+        followedTopics: const [],
+        savedHeadlines: const [],
+      );
+      await _userContentPreferencesRepository.create(
+        item: defaultPreferences,
+        userId: userId,
+      );
+      print(
+        '[DemoDataInitializerService] Default UserContentPreferences created '
+        'for user ID: $userId.',
+      );
+    } catch (e, s) {
+      print(
+        '[DemoDataInitializerService] Error ensuring UserContentPreferences '
+        'exist for user ID: $userId: $e\n$s',
+      );
+      rethrow;
+    }
+  }
+
+  /// Ensures that the [User] object for the given [user] exists in the
+  /// user client. If not found, creates it. If found, updates it.
+  ///
+  /// This is important because the `AuthInmemory` client might create a
+  /// basic user, but the `DataInMemory<User>` client might not have it
+  /// immediately.
+  Future<void> _ensureUserClientUserExists(User user) async {
+    try {
+      await _userRepository.read(id: user.id, userId: user.id);
+      // If user exists, ensure it's up-to-date (e.g., if roles changed)
+      await _userRepository.update(id: user.id, item: user, userId: user.id);
+      print(
+        '[DemoDataInitializerService] User object found and updated in '
+        'user client for ID: ${user.id}.',
+      );
+    } on NotFoundException {
+      print(
+        '[DemoDataInitializerService] User object not found in user client '
+        'for ID: ${user.id}. Creating it.',
+      );
+      await _userRepository.create(item: user, userId: user.id);
+      print(
+        '[DemoDataInitializerService] User object created in user client '
+        'for ID: ${user.id}.',
+      );
+    } catch (e, s) {
+      print(
+        '[DemoDataInitializerService] Error ensuring User object exists in '
+        'user client for ID: ${user.id}: $e\n$s',
+      );
+      rethrow;
+    }
+  }
+}