fix: convert sats to millisats when generating invoices

Critical bug fix - we were generating nearly amountless invoices.

Before:
- User enters 1000 sats in UI
- We passed 1000 to makeInvoice (thinking it's millisats)
- Generated invoice for 1000 millisats = 1 sat

After:
- User enters 1000 sats in UI
- We multiply by 1000 → 1,000,000 millisats
- Generated invoice for 1,000,000 millisats = 1000 sats ✓

This matches the send flow where we also convert sats to millisats
before passing to NWC protocol.

src/components/WalletViewer.tsx

@@ -512,15 +512,17 @@ export default function WalletViewer() {
   }
 
   async function handleGenerateInvoice() {
-    const amount = parseInt(receiveAmount);
-    if (!amount || amount <= 0) {
+    const amountSats = parseInt(receiveAmount);
+    if (!amountSats || amountSats <= 0) {
       toast.error("Please enter a valid amount");
       return;
     }
 
     setGenerating(true);
     try {
-      const result = await makeInvoice(amount, {
+      // Convert sats to millisats for NWC protocol
+      const amountMillisats = amountSats * 1000;
+      const result = await makeInvoice(amountMillisats, {
         description: receiveDescription || undefined,
       });