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/checklist_item.dart

@@ -0,0 +1,104 @@
+import 'dart:convert';
+
+/// Data model representing a single checklist item for a trip.
+///
+/// Each item stores a unique [id], the [label] to display, its completion
+/// status via [isDone], and optional timestamps to support ordering or
+/// future reminders. The model includes JSON helpers to simplify
+/// persistence with `SharedPreferences`.
+class ChecklistItem {
+  /// Unique identifier of the checklist item.
+  final String id;
+
+  /// Human‑readable text describing the task to complete.
+  final String label;
+
+  /// Indicates whether the task has been completed.
+  final bool isDone;
+
+  /// Creation timestamp used to keep a stable order in the list UI.
+  final DateTime createdAt;
+
+  /// Optional due date for the task; can be leveraged by reminders later.
+  final DateTime? dueDate;
+
+  /// Creates a checklist item.
+  const ChecklistItem({
+    required this.id,
+    required this.label,
+    required this.isDone,
+    required this.createdAt,
+    this.dueDate,
+  });
+
+  /// Builds a new item in the pending state with the current timestamp.
+  factory ChecklistItem.newItem({
+    required String id,
+    required String label,
+    DateTime? dueDate,
+  }) {
+    return ChecklistItem(
+      id: id,
+      label: label,
+      isDone: false,
+      createdAt: DateTime.now().toUtc(),
+      dueDate: dueDate,
+    );
+  }
+
+  /// Creates a copy with updated fields while keeping immutability.
+  ChecklistItem copyWith({
+    String? id,
+    String? label,
+    bool? isDone,
+    DateTime? createdAt,
+    DateTime? dueDate,
+  }) {
+    return ChecklistItem(
+      id: id ?? this.id,
+      label: label ?? this.label,
+      isDone: isDone ?? this.isDone,
+      createdAt: createdAt ?? this.createdAt,
+      dueDate: dueDate ?? this.dueDate,
+    );
+  }
+
+  /// Serializes the item to JSON for storage.
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'label': label,
+      'isDone': isDone,
+      'createdAt': createdAt.toIso8601String(),
+      'dueDate': dueDate?.toIso8601String(),
+    };
+  }
+
+  /// Deserializes a checklist item from JSON.
+  factory ChecklistItem.fromJson(Map<String, dynamic> json) {
+    return ChecklistItem(
+      id: json['id'] as String,
+      label: json['label'] as String,
+      isDone: json['isDone'] as bool? ?? false,
+      createdAt: DateTime.parse(json['createdAt'] as String),
+      dueDate: json['dueDate'] != null
+          ? DateTime.tryParse(json['dueDate'] as String)
+          : null,
+    );
+  }
+
+  /// Encodes a list of checklist items to a JSON string.
+  static String encodeList(List<ChecklistItem> items) {
+    final jsonList = items.map((item) => item.toJson()).toList();
+    return json.encode(jsonList);
+  }
+
+  /// Decodes a list of checklist items from a JSON string.
+  static List<ChecklistItem> decodeList(String jsonString) {
+    final decoded = json.decode(jsonString) as List<dynamic>;
+    return decoded
+        .cast<Map<String, dynamic>>()
+        .map(ChecklistItem.fromJson)
+        .toList();
+  }
+}