feat: implement QR code check-in system for event attendees

- Add UserQRCode component to display unique QR code on user profile
- Add QRCodeScanner component for mobile camera-based scanning
- Create EventCheckInPage for organizers to scan attendee QR codes
- Add backend endpoint POST /api/events/:id/checkin for check-in
- Add check-in button to EventDetailModal for organizers
- Install qrcode.react and html5-qrcode dependencies
- Implement real-time attendee list updates
- Add organizer permission validation
- Prevent duplicate check-ins
- Mobile-optimized with back camera support

package.json

@@ -16,9 +16,11 @@
     "@tanstack/react-query": "^5.90.9",
     "date-fns": "^4.1.0",
     "exifr": "^7.1.3",
+    "html5-qrcode": "^2.3.8",
     "leaflet": "^1.9.4",
     "leaflet.heat": "^0.2.0",
     "leaflet.markercluster": "^1.5.3",
+    "qrcode.react": "^4.2.0",
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
     "react-leaflet": "^5.0.0",
@@ -32,6 +34,7 @@
     "@types/leaflet": "^1.9.21",
     "@types/leaflet.heat": "^0.2.5",
     "@types/leaflet.markercluster": "^1.5.6",
+    "@types/qrcode.react": "^1.0.5",
     "@types/react": "^19.2.2",
     "@types/react-dom": "^19.2.2",
     "@vitejs/plugin-react": "^5.1.0",