Central now resolves site Akka remoting addresses from the Sites DB table
(NodeAAddress/NodeBAddress) instead of relying on runtime RegisterSite
messages. Eliminates the race condition where sites starting before central
had their registration dead-lettered. Addresses are cached in
CentralCommunicationActor with 60s periodic refresh and on-demand refresh
when sites are added/edited/deleted via UI or CLI.
All 22 occurrences of hardcoded "system" user string replaced with
GetCurrentUserAsync() which reads the Username claim from AuthenticationState.
Affected: Instances.razor (6), Sites.razor (2), Templates.razor (11),
SharedScripts.razor (3).
