feat: add amount preview above invoice in zap QR view

Display the zap amount prominently above the invoice when showing
the QR code. This provides clear visual confirmation of what the
user is paying before they scan or copy the invoice.

Format:
- Large bold amount with k/m notation (e.g., "420", "2.1k", "100m")
- Smaller "sats" label underneath
- Positioned between QR code and invoice field

src/components/ZapWindow.tsx

@@ -510,6 +510,16 @@ export function ZapWindow({
                 </div>
               )}
 
+              {/* Amount Preview */}
+              {(selectedAmount || customAmount) && (
+                <div className="text-center">
+                  <div className="text-3xl font-bold text-foreground">
+                    {formatAmount(selectedAmount || parseInt(customAmount))}
+                  </div>
+                  <div className="text-sm text-muted-foreground">sats</div>
+                </div>
+              )}
+
               {/* Invoice */}
               <div className="space-y-2">
                 <Label>Invoice</Label>