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/proto/proto.go

@@ -77,6 +77,10 @@ const TransportHeader = "X-Transport"
 // to when the main one is out of reach.
 const FallbackHeader = "X-Fallback-Host"
 
+// MainHeader carries the server's main address beside FallbackHeader, for an
+// agent set to the direct one to fall back to the main one.
+const MainHeader = "X-Main-Host"
+
 // WSMsg is one message on an agent's WebSocket, either way. Server to agent:
 // "ctrl" (Ctrl, as a control poll would answer), "cfg" (Tunnel and
 // Transport, also a keep-alive), "in" (Frames for session SID), "gone"
@@ -91,6 +95,7 @@ type WSMsg struct {
 	Tunnel    string          `json:"tunnel,omitempty"`
 	Transport string          `json:"transport,omitempty"`
 	Fallback  string          `json:"fallback,omitempty"`
+	Main      string          `json:"main,omitempty"`
 }
 
 // Terminal / stream frame types on the session "in" channel (server -> agent).