feat: add LNURL verify polling for zap QR payment detection

Implements automatic payment completion detection for zap QR codes using
the optional LNURL verify endpoint when available.

Changes:
- Add verify URL field to LnUrlCallbackResponse type
- Add LnUrlVerifyResponse type for verify endpoint responses
- Add verifyPayment() function to poll verify endpoint
- Store verify URL when generating invoice in ZapWindow
- Poll verify endpoint every 30 seconds when QR dialog is open
- Auto-close QR dialog and show success toast when payment detected
- Add UI feedback showing "Checking for payment..." status
- Silently ignore verification errors to allow retry on next interval

The polling activates automatically when:
1. LNURL provider includes a verify URL in the callback response
2. QR code dialog is open
3. Payment hasn't been completed yet

Users see a subtle status message showing when payment verification
is actively checking vs waiting for next check interval.

src/components/ZapWindow.tsx

@@ -46,7 +46,7 @@ import {
 import { useEmojiSearch } from "@/hooks/useEmojiSearch";
 import { useProfileSearch } from "@/hooks/useProfileSearch";
 import LoginDialog from "./nostr/LoginDialog";
-import { validateZapSupport } from "@/lib/lnurl";
+import { validateZapSupport, verifyPayment } from "@/lib/lnurl";
 import {
   createZapRequest,
   serializeZapRequest,
@@ -129,6 +129,47 @@ export function ZapWindow({
     }
   }, [wallet, getInfo]);
 
+  // Poll for payment completion via LNURL verify endpoint
+  const PAYMENT_CHECK_INTERVAL = 30000; // Check every 30 seconds
+  useEffect(() => {
+    if (!verifyUrl || !showQrDialog || isPaid) return;
+
+    const checkPayment = async () => {
+      setCheckingPayment(true);
+      try {
+        const result = await verifyPayment(verifyUrl);
+
+        // If payment is settled, mark as paid and show success
+        if (result.status === "OK" && result.settled) {
+          setIsPaid(true);
+          setShowQrDialog(false);
+          const amount = selectedAmount || parseInt(customAmount);
+          toast.success(
+            `⚡ Payment received! Zapped ${amount} sats to ${recipientName}!`,
+          );
+        }
+      } catch (error) {
+        // Silently ignore errors - will retry on next interval
+        console.debug("Payment verification check failed:", error);
+      } finally {
+        setCheckingPayment(false);
+      }
+    };
+
+    // Check immediately on mount, then every 30 seconds
+    checkPayment();
+    const intervalId = setInterval(checkPayment, PAYMENT_CHECK_INTERVAL);
+
+    return () => clearInterval(intervalId);
+  }, [
+    verifyUrl,
+    showQrDialog,
+    isPaid,
+    selectedAmount,
+    customAmount,
+    recipientName,
+  ]);
+
   // Cache LNURL data for recipient's Lightning address
   const { data: lnurlData } = useLnurlCache(recipientProfile?.lud16);
 
@@ -143,6 +184,8 @@ export function ZapWindow({
   const [showLogin, setShowLogin] = useState(false);
   const [paymentTimedOut, setPaymentTimedOut] = useState(false);
   const [zapAnonymously, setZapAnonymously] = useState(false);
+  const [verifyUrl, setVerifyUrl] = useState<string>("");
+  const [checkingPayment, setCheckingPayment] = useState(false);
 
   // Editor ref and search functions
   const editorRef = useRef<MentionEditorHandle>(null);
@@ -398,6 +441,11 @@ export function ZapWindow({
       setQrCodeUrl(qrUrl);
       setInvoice(invoiceText);
 
+      // Store verify URL if available for payment polling
+      if (invoiceResponse.verify) {
+        setVerifyUrl(invoiceResponse.verify);
+      }
+
       // Step 5: Pay or show QR code
       if (useWallet && wallet && walletInfo?.methods.includes("pay_invoice")) {
         // Pay with NWC wallet with timeout
@@ -533,6 +581,20 @@ export function ZapWindow({
                 </div>
               )}
 
+              {/* Payment Verification Status */}
+              {verifyUrl && (
+                <div className="flex items-center justify-center gap-2 text-sm text-muted-foreground">
+                  {checkingPayment && (
+                    <Loader2 className="size-3 animate-spin" />
+                  )}
+                  <span>
+                    {checkingPayment
+                      ? "Checking for payment..."
+                      : "Waiting for payment (auto-checking every 30s)"}
+                  </span>
+                </div>
+              )}
+
               {/* Amount Preview */}
               {(selectedAmount || customAmount) && (
                 <div className="text-center">

src/lib/lnurl.ts

@@ -20,6 +20,14 @@ export interface LnUrlCallbackResponse {
     message?: string;
   };
   routes?: any[];
+  verify?: string; // Optional URL to poll for payment verification
+}
+
+export interface LnUrlVerifyResponse {
+  status: "OK" | "ERROR";
+  settled: boolean;
+  preimage?: string;
+  pr?: string;
 }
 
 /**
@@ -164,3 +172,44 @@ export function validateZapSupport(lnurlData: LnUrlPayResponse): void {
     throw new Error("Invalid nostrPubkey format in LNURL response");
   }
 }
+
+/**
+ * Check payment status via LNURL verify endpoint
+ * @param verifyUrl - The verify URL from LNURL callback response
+ * @returns Promise resolving to verification response
+ */
+export async function verifyPayment(
+  verifyUrl: string,
+): Promise<LnUrlVerifyResponse> {
+  try {
+    // Add timeout to prevent hanging
+    const controller = new AbortController();
+    const timeoutId = setTimeout(() => controller.abort(), 10000); // 10s timeout
+
+    const response = await fetch(verifyUrl, { signal: controller.signal });
+    clearTimeout(timeoutId);
+
+    if (!response.ok) {
+      throw new Error(
+        `Failed to verify payment: ${response.status} ${response.statusText}`,
+      );
+    }
+
+    const data = (await response.json()) as LnUrlVerifyResponse;
+
+    // Validate response format
+    if (data.status !== "OK" && data.status !== "ERROR") {
+      throw new Error("Invalid verify response: missing or invalid status");
+    }
+
+    return data;
+  } catch (error) {
+    if (error instanceof Error) {
+      if (error.name === "AbortError") {
+        throw new Error("Payment verification timed out. Please try again.");
+      }
+      throw error;
+    }
+    throw new Error(`Failed to verify payment: ${error}`);
+  }
+}