Get all remote FPPs

Returns the list of known remote FPP systems from fppd multiSync discovery, keyed by address. Addresses that can't serve as a useful command/plugin target are filtered out: - loopback (127.0.0.0/8, ::1) — the local box discovering itself. - IPv6 link-local (fe80::/10) — needs a host-specific zone id and can't be unicast by fppd anyway (MultiSync::SendUnicastPacket is IPv4-only). - IPv4 link-local / APIPA (169.254.0.0/16) — dropped when the same device also has a routable address; kept only when it is all the device has, so a link-local-only host stays selectable. Multiple routable addresses for one device are left intact.