iap: add loading spinner to purchase actions

Apple In-app purchases do not respond immediately. This commit adds a
loading spinner element, to improve the UX and avoid the appearance of a
broken/unresponsive element.

Testing
--------

Device: iPhone 15 simulator
iOS: 17.2
Damus: This commit
Setup:
- Using the sandbox environment
- No tx to start with
- IAP experimental support enabled on developer settings in Damus
Steps:
1. Click "purchase" in any option on the damus purple screen
2. It should show a loading spinner while the purchase action is loading. PASS
3. On the confirmation screen, cancel the purchase.
4. Purchase buttons should show again. PASS

Closes: https://github.com/damus-io/damus/issues/2020
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
Link: 20240223212945.37384-1-daniel@daquino.me
Signed-off-by: William Casarin <jb55@jb55.com>

damus/Views/Purple/Detail/IAPProductStateView.swift

@@ -21,8 +21,19 @@ extension DamusPurpleView {
         let subscribe: (Product) async throws -> Void
         
         @State var show_manage_subscriptions = false
+        @State var subscription_purchase_loading = false
         
         var body: some View {
+            if subscription_purchase_loading {
+                HStack(spacing: 10) {
+                    Text(NSLocalizedString("Purchasing", comment: "Loading label indicating the purchase action is in progress"))
+                        .foregroundStyle(.white)
+                    ProgressView()
+                        .progressViewStyle(.circular)
+                        .tint(.white)
+                }
+            }
+            else {
                 switch self.products {
                     case .failed:
                         PurpleViewPrimitives.ProductLoadErrorView()
@@ -37,6 +48,7 @@ extension DamusPurpleView {
                             .progressViewStyle(.circular)
                 }
             }
+        }
         
         func PurchasedView(_ purchased: PurchasedProduct) -> some View {
             return Group {
@@ -107,7 +119,9 @@ extension DamusPurpleView {
                     Button(action: {
                         Task { @MainActor in
                             do {
+                                subscription_purchase_loading = true
                                 try await subscribe(product)
+                                subscription_purchase_loading = false
                             } catch {
                                 print(error.localizedDescription)
                             }