feat: Add services for managing trip-related data

- Implement EmergencyService for handling emergency contacts per trip.
- Create GuestFlagService to manage guest mode flags for trips.
- Introduce NotificationService with local notification capabilities.
- Add OfflineFlagService for managing offline caching flags.
- Develop PackingService for shared packing lists per trip.
- Implement ReminderService for managing reminders/to-dos per trip.
- Create SosService for dispatching SOS events to a backend.
- Add StorageService with album image upload functionality.
- Implement TransportService for managing transport segments per trip.
- Create TripChecklistService for storing and retrieving trip checklists.
- Add TripDocumentService for persisting trip documents metadata.

test: Add unit tests for new services

- Implement tests for AlbumService, BudgetService, EmergencyService, GuestFlagService, PackingService, ReminderService, SosService, TransportService, TripChecklistService, and TripDocumentService.
- Ensure tests cover adding, loading, deleting, and handling corrupted payloads for each service.

lib/services/ai_activity_service.dart

@@ -0,0 +1,59 @@
+import 'dart:convert';
+import 'package:http/http.dart' as http;
+
+/// Provides AI-powered activity suggestions using an external endpoint.
+///
+/// The endpoint is expected to accept POST JSON:
+/// { "city": "...", "interests": ["food","art"], "budget": "low|mid|high" }
+/// and return { "suggestions": [ { "title": "...", "detail": "..." }, ... ] }
+///
+/// This class is a thin client and can be wired to a custom backend that
+/// proxies LLM calls (to avoid shipping secrets in the app).
+class AiActivityService {
+  /// Base URL of the AI suggestion endpoint.
+  final String baseUrl;
+
+  /// Optional API key if your backend requires it.
+  final String? apiKey;
+
+  /// Creates an AI activity service client.
+  const AiActivityService({required this.baseUrl, this.apiKey});
+
+  /// Fetches suggestions for the given [city] with optional [interests] and [budget].
+  ///
+  /// Returns a list of string suggestions. In case of error, returns an empty list
+  /// to keep the UI responsive.
+  Future<List<String>> fetchSuggestions({
+    required String city,
+    List<String> interests = const [],
+    String budget = 'mid',
+  }) async {
+    final uri = Uri.parse('$baseUrl/ai/suggestions');
+    try {
+      final response = await http.post(
+        uri,
+        headers: {
+          'Content-Type': 'application/json',
+          if (apiKey != null) 'Authorization': 'Bearer $apiKey',
+        },
+        body: json.encode({
+          'city': city,
+          'interests': interests,
+          'budget': budget,
+        }),
+      );
+
+      if (response.statusCode == 200) {
+        final data = json.decode(response.body) as Map<String, dynamic>;
+        final list = (data['suggestions'] as List<dynamic>? ?? [])
+            .cast<Map<String, dynamic>>();
+        return list
+            .map((item) => item['title'] as String? ?? 'Suggestion')
+            .toList();
+      }
+      return const [];
+    } catch (_) {
+      return const [];
+    }
+  }
+}