fix(pairing): default relay to the transport's nostrrelay, not nostrclient proxy

The nostrclient endpoint is a subscription MULTIPLEXER, not a full relay: its
router forwards a client's EVENT upstream but never returns an OK ack (see
nostrclient/router.py). A transport client that awaits OK on publish therefore
times out ("publish timed out"), so kind-21000 RPCs never complete — verified
on the Sintra: connect succeeded but list_wallets hung, and switching to the
nostrrelay endpoint made the whole flow work (wallet, balance, availability).

default_relay_endpoint now derives from settings.nostr_transport_relays — the
relay the transport actually listens on: use it as-is when already
machine-reachable, or re-home its path on lnbits_baseurl when it's a co-located
loopback relay (the bundled nostrrelay). Validation/localhost-reject and the
pair-dialog pre-fill/hint carry over; wording updated to "transport relay".

227 tests pass.

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

templates/spirekeeper/index.html

@@ -868,7 +868,7 @@
           <q-input
             v-model="pairDialog.relays"
             label="Relay(s) for the spire's events"
-            hint="Pre-filled with this instance's nostrclient relay endpoint (derived from your Base URL). Leave as-is unless the spire must use a different relay. One per line; blank uses the default."
+            hint="Pre-filled with the relay this instance's nostr transport listens on. Leave as-is unless the spire must use a different relay. One per line; blank uses the default."
             type="textarea" autogrow
             class="q-mb-md"
             dense outlined></q-input>