feat: QR code on printed shopping lists

Added the qrcode package (server-side data-URL generation, no client
JS) rather than hand-rolling QR encoding. Only rendered when the list
is public — a private list's /s/ link 404s for anyone who isn't the
owner, so a QR code there would just be a dead scan.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/messages/en.json

@@ -908,6 +908,8 @@
   "shoppingLists": {
     "title": "Shopping Lists",
     "subtitle": "Plan your grocery runs",
+    "qrCodeAlt": "QR code linking to this shopping list",
+    "qrCodeCaption": "Scan to view online",
     "empty": "No shopping lists yet",
     "emptyDescription": "Generate one from a meal plan, or start one from scratch.",
     "listEmpty": "Empty",