feat: move remote desktop (RDP/VNC/Telnet) into a first-party plugin

Relocates guacamole backend/frontend source and tests into
plugins/remote-desktop, following the ssh-terminal/docker pattern.

plugins/remote-desktop/backend/jump-tunnel-endpoint.ts

@@ -0,0 +1,15 @@
+const LOOPBACK_HOSTS = new Set(["localhost", "127.0.0.1", "::1"]);
+
+export function resolveJumpTunnelEndpoint(
+  guacdHost: string,
+  tunnelHost = process.env.GUACD_TUNNEL_HOST,
+): { bindHost: string; advertisedHost: string } {
+  if (LOOPBACK_HOSTS.has(guacdHost.toLowerCase())) {
+    return { bindHost: "127.0.0.1", advertisedHost: "127.0.0.1" };
+  }
+
+  return {
+    bindHost: "0.0.0.0",
+    advertisedHost: tunnelHost?.trim() || "termix",
+  };
+}