feat: installer fetched from the direct address sets it as the agent's own with the main one as fallback (and downloads from it); agents fall back to whichever address is not theirs; (direct) beside the connection status

internal/server/server.go

@@ -60,6 +60,7 @@ type Server struct {
 	meters     speedMeters       // byte counts of the speed tests under way
 	xfers      xfers             // files on their way between a browser and an agent
 	ship       shipping          // files on their way from one machine to another
+	viaDirectA sync.Map          // agent id -> true while it reaches the server at the direct address
 	challenges challenges        // passkey challenges handed out
 	wsAgents   wsAgents          // agents on their WebSocket now
 	pinMisses  pinMisses         // wrong PINs per session