refactor: Replace external QR code dependency with bundled qrcode package

- Install qrcode npm package for proper QR code generation
- Create new Stimulus controller using qrcode library instead of external CDN
- Update ticket show view to use self-contained QR code generation
- Remove dependency on external qrserver.com API
- Generate valid, scannable QR codes client-side

🤖 Generated with [Claude Code](https://claude.ai/code)

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

app/javascript/controllers/index.js

@@ -18,3 +18,6 @@ application.register("ticket-selection", TicketSelectionController);
 
 import HeaderController from "./header_controller";
 application.register("header", HeaderController);
+
+import QrCodeController from "./qr_code_controller";
+application.register("qr-code", QrCodeController);