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>

views_api.py

@@ -33,6 +33,7 @@ from .pairing import (
     PairResult,
     PairingError,
     RevokeResult,
+    default_relay_endpoint,
     pair_spire,
     revoke_spire,
 )
@@ -297,6 +298,16 @@ async def api_create_machine(
     return machine
 
 
+@spirekeeper_api_router.get("/api/v1/dca/default-relay")
+async def api_default_relay(user: User = Depends(check_user_exists)) -> dict:
+    """The relay a pairing seed defaults to when the operator leaves it blank —
+    this lnbits' nostrclient proxy endpoint, derived from lnbits_baseurl
+    (bitspire#70). The pair dialog pre-fills it. `None` if lnbits_baseurl is
+    unset."""
+    _ = user
+    return {"relay": default_relay_endpoint()}
+
+
 @spirekeeper_api_router.post(
     "/api/v1/dca/machines/{machine_id}/pair", response_model=PairResult
 )