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/Models/Notification.swift

@@ -90,3 +90,7 @@ struct NotificationMarkAllReadResponse: Decodable {
     let success: Bool
     let markedCount: Int
 }
+
+struct NotificationUnreadCountResponse: Decodable {
+    let count: Int
+}