Add notification badge count and MainTabView with notification tab FRE-4740 FRE-4739

- Add getUnreadCount() endpoint to NotificationsServiceProtocol
- Add NotificationUnreadCountResponse model
- Add badgeCount and fetchUnreadCount() to NotificationsViewModel
- Update markAsRead/markAllAsRead to decrement badge count
- Create MainTabView with Home, Challenges, Clubs, Notifications tabs
- Add unread badge on notification tab using .badge() modifier
- Support injected ViewModel in NotificationsView for shared state
- Add badge count tests to NotificationServiceTests
- Fetch unread count on app launch and tab switch

Co-Authored-By: Paperclip <noreply@paperclip.ing>

Lendair/Views/NotificationsView.swift

@@ -1,8 +1,12 @@
 import SwiftUI
 
 struct NotificationsView: View {
-    @StateObject private var viewModel = NotificationsViewModel()
+    @StateObject private var viewModel: NotificationsViewModel
     @State private var showingRefreshIndicator = false
+
+    init(viewModel: NotificationsViewModel = NotificationsViewModel()) {
+        self._viewModel = StateObject(wrappedValue: viewModel)
+    }
     
     var body: some View {
         NavigationView {