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/ViewModels/NotificationsViewModel.swift

@@ -5,6 +5,7 @@ import SwiftUI
 class NotificationsViewModel: ObservableObject {
     @Published var notifications: [NotificationItem] = []
     @Published var isLoading: Bool = false
+    @Published var badgeCount: Int = 0
     @Published var lastRefreshDate: Date?
     @Published var error: NotificationError?
 
@@ -25,6 +26,7 @@ class NotificationsViewModel: ObservableObject {
         do {
             let fetchedNotifications = try await notificationsService.list()
             notifications = fetchedNotifications.sorted { $0.createdAt > $1.createdAt }
+            badgeCount = notifications.filter { !$0.isRead }.count
         } catch let error as NotificationError {
             self.error = error
         } catch {
@@ -36,12 +38,22 @@ class NotificationsViewModel: ObservableObject {
         await fetchNotifications()
     }
 
+    func fetchUnreadCount() async {
+        do {
+            let count = try await notificationsService.getUnreadCount()
+            badgeCount = count
+        } catch {
+            print("Failed to fetch unread count: \(error)")
+        }
+    }
+
     func markAsRead(id: String) async {
         guard let index = notifications.firstIndex(where: { $0.id == id }) else { return }
 
         do {
             try await notificationsService.markAsRead(id: id)
             notifications[index].isRead = true
+            badgeCount = max(0, badgeCount - 1)
             objectWillChange.send()
         } catch {
             print("Failed to mark notification as read: \(error)")
@@ -57,6 +69,7 @@ class NotificationsViewModel: ObservableObject {
             for index in notifications.indices {
                 notifications[index].isRead = true
             }
+            badgeCount = 0
             objectWillChange.send()
         } catch {
             print("Failed to mark all as read: \(error)")