Tip jar: finish transactions + Transaction.updates listener

The £4.99 consumable was never finished — StoreKit kept re-delivering
it through Transaction.updates on every launch. Verified purchases now
call transaction.finish() (direct path), and a Transaction.updates
listener finishes + acknowledges purchases that complete outside the
direct call (Ask to Buy approvals, other-device purchases on the same
Apple ID). Untrusted (unverified) transactions are never finished or
delivered; only our own product ID is handled.

FuelBoard/SettingsView.swift

@@ -564,6 +564,33 @@ final class TipStore: ObservableObject {
     @Published private(set) var purchaseInProgress = false
     @Published private(set) var message: String?
 
+    /// Listens for transactions that complete OUTSIDE the direct purchase()
+    /// call — Ask to Buy approvals, payments finished on another device
+    /// signed into the same Apple ID. Without this, those purchases land in
+    /// the queue but are never finished or acknowledged.
+    private var updatesTask: Task<Void, Never>?
+
+    init() {
+        updatesTask = Task { [weak self] in
+            for await update in Transaction.updates {
+                await self?.handle(update)
+            }
+        }
+    }
+
+    deinit {
+        updatesTask?.cancel()
+    }
+
+    private func handle(_ update: VerificationResult<StoreKit.Transaction>) async {
+        // Never deliver or finish an untrusted purchase.
+        guard case .verified(let transaction) = update else { return }
+        // Only acknowledge our own consumable (future products get their own).
+        guard transaction.productID == Self.productID else { return }
+        await transaction.finish()
+        message = NSLocalizedString("Thank you! Your tip has been received. ⛽", comment: "")
+    }
+
     var displayPrice: String {
         product?.displayPrice ?? "£4.99"
     }
@@ -599,7 +626,10 @@ final class TipStore: ObservableObject {
             switch result {
             case .success(let verification):
                 switch verification {
-                case .verified:
+                case .verified(let transaction):
+                    // Consume the consumable — otherwise StoreKit re-delivers
+                    // it through Transaction.updates on every launch.
+                    await transaction.finish()
                     message = NSLocalizedString("Thank you! Your tip has been received. ⛽", comment: "")
                 case .unverified:
                     message = NSLocalizedString("The purchase couldn't be verified. Please try again.", comment: "")