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/budget_service.dart

@@ -0,0 +1,72 @@
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/models/budget_category.dart';
+
+/// Service to manage per-trip budget envelopes (multi-devise) locally.
+///
+/// Stores envelopes in `SharedPreferences` under `budget_<tripId>` so they
+/// remain available offline. Integration with expense data can later update
+/// the [spent] field.
+class BudgetService {
+  /// Loads budget categories for the trip.
+  Future<List<BudgetCategory>> loadBudgets(String tripId) async {
+    final prefs = await SharedPreferences.getInstance();
+    final raw = prefs.getString(_key(tripId));
+    if (raw == null) return const [];
+    try {
+      return BudgetCategory.decodeList(raw);
+    } catch (_) {
+      await prefs.remove(_key(tripId));
+      return const [];
+    }
+  }
+
+  /// Persists full list.
+  Future<void> saveBudgets(
+    String tripId,
+    List<BudgetCategory> categories,
+  ) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_key(tripId), BudgetCategory.encodeList(categories));
+  }
+
+  /// Adds an envelope.
+  Future<List<BudgetCategory>> addBudget(
+    String tripId,
+    BudgetCategory category,
+  ) async {
+    final current = await loadBudgets(tripId);
+    final updated = [...current, category];
+    await saveBudgets(tripId, updated);
+    return updated;
+  }
+
+  /// Deletes an envelope.
+  Future<List<BudgetCategory>> deleteBudget(
+    String tripId,
+    String categoryId,
+  ) async {
+    final current = await loadBudgets(tripId);
+    final updated = current.where((c) => c.id != categoryId).toList();
+    await saveBudgets(tripId, updated);
+    return updated;
+  }
+
+  /// Updates spent amount for a category (used later by expense sync).
+  Future<List<BudgetCategory>> updateSpent(
+    String tripId,
+    String categoryId,
+    double spent,
+  ) async {
+    final current = await loadBudgets(tripId);
+    final updated = current
+        .map((c) {
+          if (c.id != categoryId) return c;
+          return c.copyWith(spent: spent);
+        })
+        .toList(growable: false);
+    await saveBudgets(tripId, updated);
+    return updated;
+  }
+
+  String _key(String tripId) => 'budget_$tripId';
+}