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-filesystem.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from "vitest";
+import { describeUploadError } from "./guacamole-filesystem";
+
+const t = (key: string) => `#${key}`;
+
+describe("describeUploadError", () => {
+  it("explains guacd's refusal to open the target file as a drive-folder problem", () => {
+    expect(describeUploadError(new Error("FAIL (CANNOT OPEN)"), t)).toBe(
+      "#driveNotWritable",
+    );
+    expect(describeUploadError(new Error("FAIL (NO FS)"), t)).toBe(
+      "#driveUnavailable",
+    );
+  });
+
+  it("passes other messages through and falls back for unknown errors", () => {
+    expect(describeUploadError(new Error("disk full"), t)).toBe("disk full");
+    expect(describeUploadError(undefined, t)).toBe("#uploadFailed");
+  });
+});