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/index.mjs

@@ -0,0 +1,28 @@
+/**
+ * Frontend half of the remote-desktop plugin.
+ *
+ * rdp/vnc/telnet are built-in tabs owned by this plugin (see
+ * BUILT_IN_TABS_BY_PLUGIN in src/ui/shell/pluginLoader.ts), the same
+ * category ssh-terminal's "terminal" tab is in: they render through
+ * tabUtils.tsx's hardcoded switch rather than through registerTabComponent,
+ * because GuacamoleApp needs hostId/tabId/protocol/quickConnectHost/ref --
+ * more props than the generic { tab } plugin surface can carry. There is
+ * nothing to register or unregister here; isTabTypeAvailable("rdp"/"vnc"/
+ * "telnet") is what actually gates them off when this plugin is disabled.
+ *
+ * This file exists only so the plugin has a frontend entry point matching
+ * every other plugin's shape, ready for the day a real frontend loader
+ * calls register()/unregister() directly instead of pluginLoader.ts's
+ * hand-duplicated stopgap wiring.
+ */
+
+export const id = "remote-desktop";
+export const tabIds = ["rdp", "vnc", "telnet"];
+
+export async function register() {
+  // No-op: see module comment.
+}
+
+export async function unregister() {
+  // No-op: see module comment.
+}