fix: add HTTP polling fallback for mint quote payment detection

rust/src/api/wallet.rs

@@ -234,14 +234,64 @@ impl Wallet {
             let expired_amount = quote.amount;
             let expired_expiry = quote.expiry;
 
+            // Detect payment via two parallel paths:
+            // - Path A: WebSocket (NUT-17) — fast when it works (sats on most mints)
+            // - Path B: HTTP polling every 5s — fallback for mints that don't send
+            //   WebSocket notifications for all units (e.g. Nutshell 0.20.0 + USD)
+            // First one to detect payment wins.
+            let quote_id_for_poll = quote.id.clone();
+            let poll_wallet = _self.clone();
+
             let result = tokio::time::timeout(timeout_dur, async {
+                // Enum to unify both detection paths
+                enum Detected {
+                    Paid,
+                    Issued,
+                }
+
+                let detected = tokio::select! {
+                    // Path A: WebSocket subscription
+                    result = async {
                         while let Some(event) = subscription.recv().await {
                             match event.into_inner() {
                                 NotificationPayload::MintQuoteBolt11Response(info)
                                     if info.state == CdkMintQuoteState::Paid =>
                                 {
-                            info!("Mint quote {} paid via subscription", quote.id);
+                                    info!("Mint quote {} paid via WebSocket", quote.id);
+                                    return Detected::Paid;
+                                }
+                                NotificationPayload::MintQuoteBolt11Response(info)
+                                    if info.state == CdkMintQuoteState::Issued =>
+                                {
+                                    return Detected::Issued;
+                                }
+                                _ => continue,
+                            }
+                        }
+                        // Subscription closed without detecting payment — wait for poll path
+                        std::future::pending::<Detected>().await
+                    } => result,
 
+                    // Path B: HTTP polling fallback
+                    result = async {
+                        loop {
+                            tokio::time::sleep(Duration::from_secs(5)).await;
+                            match poll_wallet.inner.check_mint_quote_status(&quote_id_for_poll).await {
+                                Ok(q) if q.state == CdkMintQuoteState::Paid => {
+                                    info!("Mint quote {} paid via HTTP polling", quote_id_for_poll);
+                                    return Detected::Paid;
+                                }
+                                Ok(q) if q.state == CdkMintQuoteState::Issued => {
+                                    return Detected::Issued;
+                                }
+                                _ => continue,
+                            }
+                        }
+                    } => result,
+                };
+
+                match detected {
+                    Detected::Paid => {
                         // Notify Dart: payment detected
                         let _ = sink.add(MintQuote {
                             id: quote.id.clone(),
@@ -304,13 +354,9 @@ impl Wallet {
                                 });
                             }
                         }
-                            return;
                     }
-                        NotificationPayload::MintQuoteBolt11Response(info)
-                            if info.state == CdkMintQuoteState::Issued =>
-                        {
-                            // Already issued (recovered from previous session) — notify Dart
-                            // so it can clean up pending metadata and show success UI
+                    Detected::Issued => {
+                        // Already issued (recovered from previous session)
                         let _ = sink.add(MintQuote {
                             id: quote.id.clone(),
                             request: quote.request.clone(),
@@ -322,9 +368,6 @@ impl Wallet {
                             transaction_id: None,
                         });
                         _self.update_balance_streams().await;
-                            return;
-                        }
-                        _ => continue,
                     }
                 }
             })