feat(pairing-ui): pre-fill the pair dialog relay with the default endpoint

The backend now defaults an omitted relay to the nostrclient proxy endpoint,
but the pair dialog still showed an empty, client-side-required field — so the
operator saw "no default". Wire it through:

- GET /api/v1/dca/default-relay returns default_relay_endpoint() (the derived
  ws(s)://<host>/nostrclient/api/v1/relay).
- openPairDialog pre-fills the relay textarea with it; the operator can override
  or clear it (blank → same server-side default). Drops the client-side
  "at least one relay is required" guard.
- Hint updated to explain the default.

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="One per line. The same relay the spire publishes to (its VITE_RELAY_URL), e.g. wss://your-host/nostrrelay/<id>"
+            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."
             type="textarea" autogrow
             class="q-mb-md"
             dense outlined></q-input>