feat(federation): presence-signing overlay on discovery enable

Enabling Nostr discoverability now opens a signer overlay (same visual
language as the app NIP-07 identity picker) before anything is published:
a locked signer row fixed to the node's discovery key — deliberately not
pickable, personal identities are never used — plus the exact signed
content (DID, npub, software version, kind 30078/NIP-33 format) and a
Sign & Publish confirm. Disabling stays immediate. Also seeds the mock
with discovery OFF to match the production default, and defaults
dwnSyncLabel to 'Unknown' when the backend omits sync_status.

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

neode-ui/mock-backend.js

@@ -228,7 +228,9 @@ function seedMockState() {
   return {
     analyticsEnabled: false,
     nodeVisibility: 'discoverable',
-    nostrDiscovery: true,
+    // Discovery starts OFF, matching the production default — enabling it in
+    // the UI walks through the presence-signing overlay.
+    nostrDiscovery: false,
     pendingPeerRequests: [
       {
         id: 'preq-demo-1',