feat: route wallet traffic through the Temetro Network relay (v0.7.0)

Devices no longer connect to the backend directly. The /wallet Socket.io
namespace is removed from realtime.ts; a new services/relay-client.ts connects
to the standalone Temetro Network relay's /hub namespace (RELAY_TOKEN-auth),
emitToWallet delegates to its sendToWallet, and device responses + wallet:online
replay are handled there via the same wallet-share/wallet-updates services.

- Add RELAY_URL + RELAY_TOKEN env (env.ts, .env.example, docker-compose.yml);
  the wallet-import QR (resolveRelayUrl) now points at RELAY_URL.
- Add socket.io-client dependency.
- Document the Temetro Network folder/service in root + backend CLAUDE.md.
- Bump root/backend/frontend to 0.7.0; CHANGELOG entry.

The relay service itself lives in github.com/temetro/temetro-network.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "frontend",
-  "version": "0.6.0",
+  "version": "0.7.0",
   "private": true,
   "scripts": {
     "dev": "next dev",