feat: printable receipts with barcode on payment complete

- Receipt component with thermal (80mm) and full-page layout support
- Code 128 barcode from transaction number via JsBarcode
- Store name, address, line items, totals, payment info, barcode
- Print button on sale complete screen (browser print dialog)
- Email button placeholder (disabled, ready for SMTP integration)
- @media print CSS hides everything except receipt content
- Receipt data fetched from GET /transactions/:id/receipt

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/admin/src/app.css

@@ -113,6 +113,14 @@ body {
   background-color: var(--muted-foreground);
 }
 
+/* Print styles — only show receipt content */
+@media print {
+  body * { visibility: hidden; }
+  .print\:block, .print\:block * { visibility: visible; }
+  .print\:block { position: absolute; left: 0; top: 0; }
+  .print\:hidden { display: none !important; }
+}
+
 /* Prevent browser autofill from overriding dark theme input colors */
 input:-webkit-autofill,
 input:-webkit-autofill:hover,