chore(mock): node.nostr-pubkey returns exact real shape (hex + genuine npub)

The mock previously returned a placeholder string with no nostr_npub, so the
new Federation signing-details panel showed 'loading…' forever against the
mock. Now returns a distinct discovery key — 64-char hex plus its real
NIP-19 bech32 encoding (verified against the official test vector) —
mirroring production where the node discovery key is separate from the
personal identity keys.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

neode-ui/mock-backend.js

@@ -1634,7 +1634,14 @@ app.post('/rpc/v1', (req, res) => {
         return res.json({ result: { event_id: 'mock-event-id', success: 2, failed: 0 } })
       }
       case 'node.nostr-pubkey': {
-        return res.json({ result: { nostr_pubkey: 'mock-nostr-pubkey-hex' } })
+        // Exact shape of the real handler (node.rs handle_node_nostr_pubkey):
+        // the node's dedicated discovery key — hex + genuine bech32 npub
+        // (deliberately NOT one of the personal identity keys; discovery
+        // presence events are always signed with this separate node key).
+        return res.json({ result: {
+          nostr_pubkey: 'c9f0a2e84b71d3568e0f4a6b2c8d1e97a3b5c7d9e1f2a4b6c8d0e2f4a6b8c0d2',
+          nostr_npub: 'npub1e8c296ztw8f4drs0ff4jerg7j73mt37eu8e2fdkg6r30ff4ccrfq8j5vuy',
+        } })
       }
 
       case 'node.signChallenge': {