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/transport_service.dart

@@ -0,0 +1,72 @@
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/models/transport_segment.dart';
+
+/// Service that stores per-trip transport segments locally for offline access.
+///
+/// Uses `SharedPreferences` keyed by `trip_transport_<tripId>` to keep
+/// creation/edit quick without round-trips. Real-time status can later be
+/// updated by a background job hitting external APIs.
+class TransportService {
+  /// Loads stored transport segments for [tripId].
+  Future<List<TransportSegment>> loadSegments(String tripId) async {
+    final prefs = await SharedPreferences.getInstance();
+    final raw = prefs.getString(_key(tripId));
+    if (raw == null) return const [];
+    try {
+      return TransportSegment.decodeList(raw);
+    } catch (_) {
+      await prefs.remove(_key(tripId));
+      return const [];
+    }
+  }
+
+  /// Persists the full list of segments for [tripId].
+  Future<void> saveSegments(
+    String tripId,
+    List<TransportSegment> segments,
+  ) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_key(tripId), TransportSegment.encodeList(segments));
+  }
+
+  /// Adds a segment entry.
+  Future<List<TransportSegment>> addSegment(
+    String tripId,
+    TransportSegment segment,
+  ) async {
+    final current = await loadSegments(tripId);
+    final updated = [...current, segment];
+    await saveSegments(tripId, updated);
+    return updated;
+  }
+
+  /// Deletes a segment by [segmentId].
+  Future<List<TransportSegment>> deleteSegment(
+    String tripId,
+    String segmentId,
+  ) async {
+    final current = await loadSegments(tripId);
+    final updated = current.where((s) => s.id != segmentId).toList();
+    await saveSegments(tripId, updated);
+    return updated;
+  }
+
+  /// Updates the status of a segment (e.g., delayed/boarding/in_air).
+  Future<List<TransportSegment>> updateStatus(
+    String tripId,
+    String segmentId,
+    String status,
+  ) async {
+    final current = await loadSegments(tripId);
+    final updated = current
+        .map((s) {
+          if (s.id != segmentId) return s;
+          return s.copyWith(status: status);
+        })
+        .toList(growable: false);
+    await saveSegments(tripId, updated);
+    return updated;
+  }
+
+  String _key(String tripId) => 'trip_transport_$tripId';
+}