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/activity_suggestion_service.dart

@@ -0,0 +1,39 @@
+import 'dart:math';
+
+/// Provides lightweight, offline activity suggestions using heuristics.
+class ActivitySuggestionService {
+  /// Returns a list of suggestion strings based on [city] and [weatherCode].
+  ///
+  /// [weatherCode] is a simple tag: `sunny`, `rain`, `cold`, `default`.
+  List<String> suggestions({
+    required String city,
+    String weatherCode = 'default',
+  }) {
+    final base = <String>[
+      'Free walking tour de $city',
+      'Spot photo au coucher du soleil',
+      'Café local pour travailler/charger',
+      'Parc ou rooftop tranquille',
+    ];
+
+    if (weatherCode == 'rain') {
+      base.addAll([
+        'Musée immanquable de $city',
+        'Escape game ou activité indoor',
+        'Food court couvert pour goûter local',
+      ]);
+    } else if (weatherCode == 'cold') {
+      base.addAll(['Spa / bains chauds', 'Visite guidée en intérieur']);
+    } else {
+      base.addAll([
+        'Balade vélo ou trottinette',
+        'Pique-nique au parc central',
+        'Vue panoramique / rooftop',
+      ]);
+    }
+
+    // Shuffle slightly for variation.
+    base.shuffle(Random(city.hashCode));
+    return base.take(6).toList();
+  }
+}