feat: Add PDF download button for car images in admin car detail modal

- Download all car images as a single PDF (A4 landscape, one image per page)
- Button shows image count and displays in modal header for easy access
- Uses jsPDF library for client-side PDF generation

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

frontend/package.json

@@ -17,6 +17,7 @@
     "@types/leaflet": "^1.9.21",
     "axios": "^1.6.5",
     "framer-motion": "^12.23.25",
+    "jspdf": "^4.2.1",
     "leaflet": "^1.9.4",
     "next": "14.1.0",
     "react": "^18.2.0",