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/MainTabView.swift

@@ -0,0 +1,79 @@
+import SwiftUI
+
+struct MainTabView: View {
+    @State private var selectedTab: AppTab = .home
+    @StateObject private var notificationVM = NotificationsViewModel()
+
+    var body: some View {
+        TabView(selection: $selectedTab) {
+            Group {
+                TrainingPlanView()
+                    .tag(AppTab.home)
+                    .tabItem {
+                        Label(AppTab.home.title, systemImage: AppTab.home.icon)
+                    }
+
+                ChallengesView()
+                    .tag(AppTab.challenges)
+                    .tabItem {
+                        Label(AppTab.challenges.title, systemImage: AppTab.challenges.icon)
+                    }
+
+                ClubsView()
+                    .tag(AppTab.clubs)
+                    .tabItem {
+                        Label(AppTab.clubs.title, systemImage: AppTab.clubs.icon)
+                    }
+
+                NotificationsView(viewModel: notificationVM)
+                    .tag(AppTab.notifications)
+                    .tabItem {
+                        Label(AppTab.notifications.title, systemImage: AppTab.notifications.icon)
+                    }
+                    .badge(notificationVM.badgeCount)
+            }
+        }
+        .onAppear {
+            Task {
+                await notificationVM.fetchUnreadCount()
+            }
+        }
+        .onChange(of: selectedTab) { _, newTab in
+            if newTab == .notifications {
+                Task {
+                    await notificationVM.fetchNotifications()
+                    await notificationVM.fetchUnreadCount()
+                }
+            }
+        }
+    }
+}
+
+enum AppTab: String, CaseIterable {
+    case home
+    case challenges
+    case clubs
+    case notifications
+
+    var title: String {
+        switch self {
+        case .home: return "Home"
+        case .challenges: return "Challenges"
+        case .clubs: return "Clubs"
+        case .notifications: return "Notifications"
+        }
+    }
+
+    var icon: String {
+        switch self {
+        case .home: return "house.fill"
+        case .challenges: return "flag.fill"
+        case .clubs: return "person.3.fill"
+        case .notifications: return "bell.fill"
+        }
+    }
+}
+
+#Preview {
+    MainTabView()
+}