feat(signer): QR pairing — client-initiated nostrconnect:// flow for Amber

Keynctr is the NIP-46 client; Amber is the scanner. Amber hands out no
link — it scans one — so the signer screen now mints a pairing token:

- start_pairing(): ephemeral key + secret, nostrconnect:// token via
  NostrConnectUri::client_with_secret, status().pairing_uri for the GUI
- run_pairing_task(): listens for the signer's connect request, echoes
  the secret (anti-spoofing), persists the connection row + secret,
  then adopts identity via get_public_key and hands to the demux loop
- pairing subscription is closed at handoff so the demux loop owns the
  conversation (relay could otherwise deliver signer replies under the
  stale pairing sub id where nobody routes them)
- IPC: nip46_pair_start; status carries pairing_uri
- SignerModeScreen: 'Show QR' button, QR render (qrcode) of the token,
  copy-link fallback, cancel; paste-link flow unchanged
- e2e: fake QR scanner consumes the real pairing token end-to-end
  (scan -> secret echo -> identity -> sign -> vault persistence)

frontend/package.json

@@ -28,6 +28,7 @@
   },
   "dependencies": {
     "nostr-tools": "^2.25.1",
+    "qrcode": "^1.5.4",
     "react": "^18.3.1",
     "react-dom": "^18.3.1"
   },
@@ -38,6 +39,7 @@
     "@testing-library/react": "^16.1.0",
     "@testing-library/user-event": "^14.5.2",
     "@types/node": "^26.1.2",
+    "@types/qrcode": "^1.5.6",
     "@types/react": "^18.3.12",
     "@types/react-dom": "^18.3.1",
     "@vitejs/plugin-react": "^6.1.0",