fix: dismiss paywall and sync premium state after successful purchase

- Add purchaseSucceeded flag to PurchaseViewModel, set on purchase success
- PaywallView observes the flag and dismisses itself automatically
- ProfileTab.syncSubscription() writes PurchaseService.currentPlan back
  to UserProfile.subscriptionRaw via SwiftData on sheet dismiss

tabatago-swift/TabataGo/ViewModels/PurchaseViewModel.swift

@@ -7,6 +7,7 @@ final class PurchaseViewModel: ObservableObject {
     @Published var offerings: Offerings? = nil
     @Published var selectedPackage: Package? = nil
     @Published var isPurchasing = false
+    @Published var purchaseSucceeded = false
     @Published var showError = false
     @Published var errorMessage: String? = nil
 
@@ -27,6 +28,7 @@ final class PurchaseViewModel: ObservableObject {
         do {
             try await service.purchase(package: package)
             AnalyticsService.shared.subscriptionStarted(plan: package.identifier)
+            purchaseSucceeded = true
         } catch {
             errorMessage = error.localizedDescription
             showError = true