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/guacamole-file-drop.ts

@@ -0,0 +1,22 @@
+export type FileDropDisposition = "ignore" | "reject" | "upload";
+
+export function hasDraggedFiles(types: readonly string[]): boolean {
+  return types.includes("Files");
+}
+
+export function getFileDropDisposition(
+  types: readonly string[],
+  fileCount: number,
+  canUpload: boolean,
+): FileDropDisposition {
+  if (!hasDraggedFiles(types) || fileCount === 0) return "ignore";
+  return canUpload ? "upload" : "reject";
+}
+
+export function canUploadToRdpDrive(
+  uploadAllowed: boolean,
+  driveEnabled: boolean,
+  hasFilesystem: boolean,
+): boolean {
+  return uploadAllowed && (driveEnabled || hasFilesystem);
+}