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/frontend/rdp-credential-prompt.ts

@@ -0,0 +1,17 @@
+import type { HostAuthOverrides } from "@/types/auth-protocols";
+
+export function needsRdpCredentialPrompt({
+  protocol,
+  rdpAuthType,
+  authOverrides,
+}: {
+  protocol: "rdp" | "vnc" | "telnet";
+  rdpAuthType?: string;
+  authOverrides?: HostAuthOverrides;
+}): boolean {
+  return (
+    protocol === "rdp" &&
+    rdpAuthType === "none" &&
+    !authOverrides?.rdp?.credentialId
+  );
+}