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/models/reminder_item.dart

@@ -0,0 +1,106 @@
+import 'dart:convert';
+
+/// Represents a dated reminder or to-do for the trip.
+class ReminderItem {
+  /// Unique identifier.
+  final String id;
+
+  /// Text to display.
+  final String title;
+
+  /// Optional detailed note.
+  final String? note;
+
+  /// Due date/time (UTC) for the reminder.
+  final DateTime dueAt;
+
+  /// Completion flag.
+  final bool isDone;
+
+  /// Creation timestamp.
+  final DateTime createdAt;
+
+  /// Creates a reminder item.
+  const ReminderItem({
+    required this.id,
+    required this.title,
+    required this.dueAt,
+    required this.isDone,
+    required this.createdAt,
+    this.note,
+  });
+
+  /// Convenience builder for new pending reminder.
+  factory ReminderItem.newItem({
+    required String id,
+    required String title,
+    required DateTime dueAt,
+    String? note,
+  }) {
+    return ReminderItem(
+      id: id,
+      title: title,
+      note: note,
+      dueAt: dueAt,
+      isDone: false,
+      createdAt: DateTime.now().toUtc(),
+    );
+  }
+
+  /// Copy with changes.
+  ReminderItem copyWith({
+    String? id,
+    String? title,
+    String? note,
+    DateTime? dueAt,
+    bool? isDone,
+    DateTime? createdAt,
+  }) {
+    return ReminderItem(
+      id: id ?? this.id,
+      title: title ?? this.title,
+      note: note ?? this.note,
+      dueAt: dueAt ?? this.dueAt,
+      isDone: isDone ?? this.isDone,
+      createdAt: createdAt ?? this.createdAt,
+    );
+  }
+
+  /// JSON serialization.
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'title': title,
+      'note': note,
+      'dueAt': dueAt.toIso8601String(),
+      'isDone': isDone,
+      'createdAt': createdAt.toIso8601String(),
+    };
+  }
+
+  /// JSON deserialization.
+  factory ReminderItem.fromJson(Map<String, dynamic> json) {
+    return ReminderItem(
+      id: json['id'] as String,
+      title: json['title'] as String,
+      note: json['note'] as String?,
+      dueAt: DateTime.parse(json['dueAt'] as String),
+      isDone: json['isDone'] as bool? ?? false,
+      createdAt: DateTime.parse(json['createdAt'] as String),
+    );
+  }
+
+  /// Encodes list.
+  static String encodeList(List<ReminderItem> reminders) {
+    return json.encode(reminders.map((r) => r.toJson()).toList());
+  }
+
+  /// Decodes list.
+  static List<ReminderItem> decodeList(String raw) {
+    final decoded = json.decode(raw) as List<dynamic>;
+    return decoded
+        .cast<Map<String, dynamic>>()
+        .map(ReminderItem.fromJson)
+        .toList();
+  }
+}