feat: QR pairing for wallet import (backend + website)

- Backend: nullable wallet_number on wallet_share_requests (migration 0029);
  createPairingRequest + POST /api/patients/wallet/pair; applyShareResponse
  binds the authenticated wallet on QR (pairing) requests
- Frontend: Import dialog gains a "Show QR" mode rendering a temetro-pair QR
  (react-qr-code) the patient scans; requestWalletPairing helper; i18n keys

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -56,6 +56,7 @@
     "react-dom": "19.2.4",
     "react-i18next": "^17.0.8",
     "react-jsx-parser": "^2.4.1",
+    "react-qr-code": "^2.2.0",
     "shadcn": "^4.8.3",
     "shiki": "^4.1.0",
     "socket.io-client": "^4.8.3",