feat: receipt PDF save and print via html2pdf.js

- Save PDF button downloads receipt directly
- Print button opens PDF in new window and triggers print dialog
- Replaces previous window.print() approach that lost styles
- Receipt generated on demand from transaction data, no file storage needed

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

packages/admin/src/app.css

@@ -113,14 +113,6 @@ 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,