fix: default RELAY_URL to the hosted relay (v0.8.2)

The default http://localhost:8080 silently failed for clinics that joined
the network without setting RELAY_URL — inside Docker localhost is the
container itself, so the hub connection never reached the relay (endless
"relay unreachable" retries) and pairing QRs encoded an unreachable
localhost. Default to https://network.temetro.com so "Join Temetro Network"
works out of the box; self-hosters running their own relay still override
it. Also (re)ensure the hub is connected before pre-registering a pairing.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "frontend",
-  "version": "0.8.1",
+  "version": "0.8.2",
   "private": true,
   "scripts": {
     "dev": "next dev",