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>

LendairTests/NotificationServiceTests.swift

@@ -10,6 +10,8 @@ final class MockNotificationsService: NotificationsServiceProtocol {
     var markAllCalled = false
     var listCallCount = 0
     var listError: Error?
+    var mockUnreadCount: Int = 0
+    var getUnreadCountCallCount = 0
 
     func list(params: NotificationListParams = NotificationListParams()) async throws -> [NotificationItem] {
         listCallCount += 1
@@ -26,6 +28,11 @@ final class MockNotificationsService: NotificationsServiceProtocol {
     func markAllAsRead() async throws {
         markAllCalled = true
     }
+
+    func getUnreadCount() async throws -> Int {
+        getUnreadCountCallCount += 1
+        return mockUnreadCount
+    }
 }
 
 // MARK: - Helper: Sample Notifications
@@ -216,6 +223,91 @@ final class NotificationServiceTests: XCTestCase {
         XCTAssertEqual(mock.listCallCount, 1)
         XCTAssertEqual(viewModel.notifications.count, 1)
     }
+
+    // MARK: - Badge Count
+
+    @MainActor
+    func testFetchUnreadCountSetsBadgeCount() async {
+        let mock = MockNotificationsService()
+        mock.mockUnreadCount = 5
+
+        let viewModel = NotificationsViewModel(notificationsService: mock)
+        await viewModel.fetchUnreadCount()
+
+        XCTAssertEqual(viewModel.badgeCount, 5)
+        XCTAssertEqual(mock.getUnreadCountCallCount, 1)
+    }
+
+    @MainActor
+    func testFetchUnreadCountZero() async {
+        let mock = MockNotificationsService()
+        mock.mockUnreadCount = 0
+
+        let viewModel = NotificationsViewModel(notificationsService: mock)
+        await viewModel.fetchUnreadCount()
+
+        XCTAssertEqual(viewModel.badgeCount, 0)
+    }
+
+    @MainActor
+    func testMarkAsReadDecrementsBadgeCount() async {
+        let mock = MockNotificationsService()
+        let unread = NotificationItem.sample(id: "1", isRead: false)
+
+        let viewModel = NotificationsViewModel(notificationsService: mock)
+        viewModel.notifications = [unread]
+        viewModel.badgeCount = 3
+
+        await viewModel.markAsRead(id: "1")
+
+        XCTAssertEqual(viewModel.badgeCount, 2)
+        XCTAssertTrue(viewModel.notifications.first?.isRead == true)
+    }
+
+    @MainActor
+    func testMarkAsReadBadgeCountDoesNotGoBelowZero() async {
+        let mock = MockNotificationsService()
+        let unread = NotificationItem.sample(id: "1", isRead: false)
+
+        let viewModel = NotificationsViewModel(notificationsService: mock)
+        viewModel.notifications = [unread]
+        viewModel.badgeCount = 0
+
+        await viewModel.markAsRead(id: "1")
+
+        XCTAssertEqual(viewModel.badgeCount, 0)
+    }
+
+    @MainActor
+    func testMarkAllAsReadResetsBadgeCount() async {
+        let mock = MockNotificationsService()
+        let unread1 = NotificationItem.sample(id: "1", isRead: false)
+        let unread2 = NotificationItem.sample(id: "2", isRead: false)
+
+        let viewModel = NotificationsViewModel(notificationsService: mock)
+        viewModel.notifications = [unread1, unread2]
+        viewModel.badgeCount = 7
+
+        await viewModel.markAllAsRead()
+
+        XCTAssertEqual(viewModel.badgeCount, 0)
+    }
+
+    @MainActor
+    func testFetchNotificationsUpdatesBadgeCount() async {
+        let mock = MockNotificationsService()
+        mock.notifications = [
+            NotificationItem.sample(id: "1", isRead: false),
+            NotificationItem.sample(id: "2", isRead: true),
+            NotificationItem.sample(id: "3", isRead: false),
+        ]
+
+        let viewModel = NotificationsViewModel(notificationsService: mock)
+        viewModel.badgeCount = 0
+        await viewModel.fetchNotifications()
+
+        XCTAssertEqual(viewModel.badgeCount, 2)
+    }
 }
 
 // MARK: - NotificationModelTests