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>

LendairTests/ChallengeServiceTests.swift

@@ -10,6 +10,7 @@ final class MockChallengeService: ChallengeServiceProtocol {
     var joinCalledIds: [String] = []
     var leaveCalledIds: [String] = []
     var createCalled = false
+    var updateCalled = false
     var leaderboard: [LeaderboardEntry] = []
     var listCallCount = 0
     var listError: Error?
@@ -51,7 +52,28 @@ final class MockChallengeService: ChallengeServiceProtocol {
     }
 
     func updateChallenge(id: String, request: UpdateChallengeRequest) async throws -> Challenge {
-        throw ChallengeError.notFound
+        updateCalled = true
+        return Challenge(
+            id: id,
+            title: request.title ?? "Updated",
+            description: request.description ?? "",
+            challengeType: request.challengeType ?? .distance,
+            status: .active,
+            startDate: Date(),
+            endDate: Date().addingTimeInterval(30 * 24 * 3600),
+            targetMetric: request.targetMetric ?? .distance,
+            targetValue: request.targetValue ?? 100,
+            targetUnit: (request.targetMetric ?? .distance).unit,
+            participantCount: 0,
+            rules: request.rules,
+            imageUrl: nil,
+            createdBy: "current-user",
+            createdByName: "Current User",
+            clubId: nil,
+            participationStatus: .participating,
+            userProgress: 0,
+            createdAt: Date()
+        )
     }
 
     func joinChallenge(id: String) async throws {