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>

static/js/index.js

@@ -839,12 +839,21 @@ window.app = Vue.createApp({
     // -----------------------------------------------------------------
     // Pair / revoke spire (S0 / #9, #12)
     // -----------------------------------------------------------------
-    openPairDialog(machine) {
+    async openPairDialog(machine) {
       this.pairDialog.machine = machine
       this.pairDialog.relays = ''
       this.pairDialog.durationHours = null
       this.pairDialog.result = null
       this.pairDialog.show = true
+      // Pre-fill with this lnbits' default relay (the nostrclient proxy
+      // endpoint derived from lnbits_baseurl). The operator can override or
+      // clear it; if left blank the backend fills in the same default.
+      try {
+        const {data} = await LNbits.api.request('GET', `${API}/default-relay`)
+        if (data && data.relay) this.pairDialog.relays = data.relay
+      } catch (e) {
+        // Non-fatal: leave blank; the backend still defaults on submit.
+      }
     },
 
     async submitPair() {
@@ -852,14 +861,12 @@ window.app = Vue.createApp({
         .split(/[\s,]+/)
         .map(s => s.trim())
         .filter(Boolean)
-      if (!relays.length) {
-        Quasar.Notify.create({
-          type: 'negative',
-          message: 'At least one relay is required'
-        })
-        return
+      // relays is optional — blank falls back to the nostrclient endpoint
+      // server-side (the pre-filled default), so no client-side requirement.
+      const body = {}
+      if (relays.length) {
+        body.relays = relays
       }
-      const body = {relays}
       if (this.pairDialog.durationHours) {
         body.duration_hours = Number(this.pairDialog.durationHours)
       }