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/drive-settings.ts

@@ -0,0 +1,29 @@
+export const GUACD_DRIVE_PATH_ENV = "GUACD_DRIVE_PATH";
+const DEFAULT_DRIVE_ROOT = "/drive";
+
+/**
+ * Fills in where guacd keeps the files behind RDP drive redirection.
+ *
+ * The folder lives on the guacd host, not on Termix's - with the stock
+ * compose it is a directory in the shared termix-data volume, configured
+ * through GUACD_DRIVE_PATH. Each user gets a folder of their own underneath:
+ * a shared drive would show everyone's uploads to everyone else. A host that
+ * names its own drive-path keeps it.
+ */
+export function withDriveSettings(
+  guacConfig: Record<string, unknown>,
+  userId: string,
+  env: NodeJS.ProcessEnv = process.env,
+): Record<string, unknown> {
+  if (!guacConfig["enable-drive"] || guacConfig["drive-path"]) {
+    return guacConfig;
+  }
+  const root = (
+    env[GUACD_DRIVE_PATH_ENV]?.trim() || DEFAULT_DRIVE_ROOT
+  ).replace(/\/+$/, "");
+  return {
+    ...guacConfig,
+    "drive-path": `${root}/${userId}`,
+    "create-drive-path": true,
+  };
+}