Adds configurable periodic 0-hop self-advert that includes GPS coordinates,
making the device's position visible in other nodes' Nearby Nodes screen.
Options: off / 1min / 2min / 5min / 10min / 30min / 1h.
Accessible via Tools → Auto-Advert. Timer runs in MyMesh::loop().
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>