chore(fixtures): populate user content preferences demo data

Populates the `userContentPreferencesFixturesData` with realistic, interconnected data for the demo user. This includes followed topics, sources, and countries, as well as saved headlines and filters.

This provides a richer dataset for client-side development and demonstration, particularly for features related to user personalization and saved filters.

Author: fulleni

lib/src/fixtures/user_content_preferences.dart

@@ -1,14 +1,41 @@
 import 'package:core/src/fixtures/fixture_ids.dart';
+import 'package:core/src/fixtures/headlines.dart';
+import 'package:core/src/fixtures/sources.dart';
+import 'package:core/src/fixtures/topics.dart';
 import 'package:core/src/models/user_preferences/user_content_preferences.dart';
+import 'package:core/src/models/user_presets/saved_filter.dart';
 
 /// User Content Preferences Demo Data
 final List<UserContentPreferences> userContentPreferencesFixturesData = [
-  const UserContentPreferences(
+  UserContentPreferences(
     id: kAdminUserId,
-    followedCountries: [],
-    followedSources: [],
-    followedTopics: [],
-    savedHeadlines: [],
-    savedFilters: [],
+    followedCountries: const [],
+    followedSources: [
+      sourcesFixturesData[0], // TechCrunch
+      sourcesFixturesData[1], // BBC News
+    ],
+    followedTopics: [
+      topicsFixturesData[0], // Technology
+      topicsFixturesData[1], // Sports
+      topicsFixturesData[6], // Business
+      topicsFixturesData[7], // Travel
+    ],
+    savedHeadlines: [headlinesFixturesData[0], headlinesFixturesData[10]],
+    savedFilters: [
+      SavedFilter(
+        id: kSavedFilterId1,
+        name: 'Tech News',
+        topics: [topicsFixturesData[0]], // Technology
+        sources: [sourcesFixturesData[0]], // TechCrunch
+        countries: const [],
+      ),
+      SavedFilter(
+        id: kSavedFilterId2,
+        name: 'World Biz',
+        topics: [topicsFixturesData[6]], // Business
+        sources: const [],
+        countries: const [],
+      ),
+    ],
   ),
 ];