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/Services/NotificationService.swift

@@ -6,6 +6,7 @@ protocol NotificationsServiceProtocol: Sendable {
     func list(params: NotificationListParams) async throws -> [NotificationItem]
     func markAsRead(id: String) async throws
     func markAllAsRead() async throws
+    func getUnreadCount() async throws -> Int
 }
 
 // MARK: - Default Service
@@ -62,6 +63,17 @@ class NotificationsService: NotificationsServiceProtocol {
         _ = try JSONDecoder().decode(NotificationMarkAllReadResponse.self, from: data)
     }
 
+    func getUnreadCount() async throws -> Int {
+        let url = baseURL.appendingPathComponent("/api/notifications/unread-count")
+        let request = try buildRequest(url: url)
+
+        let (data, response) = try await session.data(for: request)
+        try validateResponse(response)
+
+        let decoded = try JSONDecoder().decode(NotificationUnreadCountResponse.self, from: data)
+        return decoded.count
+    }
+
     // MARK: - Helpers
 
     private func buildRequest(url: URL, method: HTTPMethod = .get, body: Data? = nil) throws -> URLRequest {