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/reminder_service.dart

@@ -0,0 +1,64 @@
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/models/reminder_item.dart';
+
+/// Persists dated reminders/to-dos per trip locally for offline use.
+class ReminderService {
+  /// Loads reminders for [tripId].
+  Future<List<ReminderItem>> loadReminders(String tripId) async {
+    final prefs = await SharedPreferences.getInstance();
+    final raw = prefs.getString(_key(tripId));
+    if (raw == null) return const [];
+    try {
+      return ReminderItem.decodeList(raw);
+    } catch (_) {
+      await prefs.remove(_key(tripId));
+      return const [];
+    }
+  }
+
+  /// Saves full list.
+  Future<void> saveReminders(
+    String tripId,
+    List<ReminderItem> reminders,
+  ) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_key(tripId), ReminderItem.encodeList(reminders));
+  }
+
+  /// Adds a reminder.
+  Future<List<ReminderItem>> addReminder(
+    String tripId,
+    ReminderItem reminder,
+  ) async {
+    final current = await loadReminders(tripId);
+    final updated = [...current, reminder];
+    await saveReminders(tripId, updated);
+    return updated;
+  }
+
+  /// Toggles done state.
+  Future<List<ReminderItem>> toggleReminder(
+    String tripId,
+    String reminderId,
+  ) async {
+    final current = await loadReminders(tripId);
+    final updated = current
+        .map((r) => r.id == reminderId ? r.copyWith(isDone: !r.isDone) : r)
+        .toList(growable: false);
+    await saveReminders(tripId, updated);
+    return updated;
+  }
+
+  /// Deletes a reminder.
+  Future<List<ReminderItem>> deleteReminder(
+    String tripId,
+    String reminderId,
+  ) async {
+    final current = await loadReminders(tripId);
+    final updated = current.where((r) => r.id != reminderId).toList();
+    await saveReminders(tripId, updated);
+    return updated;
+  }
+
+  String _key(String tripId) => 'reminders_$tripId';
+}