Fix P0-P3 code review issues for clubs and challenges (FRE-4664)

P0: Fix variable shadowing in ClubService.createClub/updateClub and
    ChallengeService.createChallenge/updateChallenge — renamed local
    'var request' to 'var urlRequest' so JSONEncoder encodes the
    typed parameter, not the URLRequest.

P1: Wire CreateClubSheet and CreateChallengeSheet to parent ViewModel —
    sheets now receive viewModel and call createClub/createChallenge
    before dismissing.

P2: Extract HTTPMethod enum to shared Utils/HTTPMethod.swift (was
    defined in NotificationService). Remove dead 'body' parameter from
    buildRequest in all three services. Add error alert UI to
    ClubsView and ChallengesView.

P3: Replace forced URL unwrap with static let defaultBaseURL in all
    three services. Fix MockChallengeService.updateChallenge to track
    updateCalled instead of always throwing notFound.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

Lendair/Views/ChallengesView.swift

@@ -4,6 +4,7 @@ struct ChallengesView: View {
     @StateObject private var viewModel = ChallengeViewModel()
     @State private var showingCreateSheet = false
     @State private var selectedTab: ChallengeTab = .active
+    @State private var lastError: ChallengeError?
 
     enum ChallengeTab: String, CaseIterable {
         case active, upcoming, completed
@@ -32,7 +33,12 @@ struct ChallengesView: View {
                 }
             }
             .sheet(isPresented: $showingCreateSheet) {
-                CreateChallengeSheet()
+                CreateChallengeSheet(viewModel: viewModel)
+            }
+            .alert("Error", isPresented: .init(get: { viewModel.error != nil }, set: { if !$0 { lastError = viewModel.error } })) {
+                Button("OK") { lastError = viewModel.error }
+            } message: {
+                Text(viewModel.error?.errorDescription ?? "")
             }
         }
         .onAppear {
@@ -116,6 +122,7 @@ struct ChallengesView: View {
 
 struct CreateChallengeSheet: View {
     @Environment(\.dismiss) var dismiss
+    let viewModel: ChallengeViewModel
     @State private var title = ""
     @State private var description = ""
     @State private var challengeType: ChallengeType = .distance
@@ -177,7 +184,10 @@ struct CreateChallengeSheet: View {
                             rules: rules.isEmpty ? nil : rules,
                             clubId: nil
                         )
-                        dismiss()
+                        Task {
+                            _ = await viewModel.createChallenge(request: request)
+                            dismiss()
+                        }
                     }
                     .disabled(title.isEmpty || targetValue.isEmpty)
                 }