Move ai-stack and paintplus vendored source under vendor/

Matches the existing vendor/easy-asterisk convention (used by
services/asterisk.sh) instead of two one-off top-level directories that
cluttered the repo root and didn't look like anything else next to
setup.sh, lib/, services/, extras/. Only the two services' own SRC_DIR
path resolution and header comments needed updating — nothing else in
the repo referenced the old ./ai-stack / ./paintplus paths.

Also documents vendor/ in README.md's Layout section.

vendor/paintplus/frontend/src/js/actions/toggle-layer-visibility.js

@@ -0,0 +1,37 @@
+import app from '../app.js';
+import config from '../config.js';
+import { Base_action } from './base.js';
+
+export class Toggle_layer_visibility_action extends Base_action {
+	/**
+	 * toggle layer visibility
+	 *
+	 * @param {int} layer_id
+	 */
+	constructor(layer_id) {
+		super('toggle_layer_visibility', 'Toggle Layer Visibility');
+		this.layer_id = parseInt(layer_id);
+		this.old_visible = null;
+	}
+
+	async do() {
+		super.do();
+		const layer = app.Layers.get_layer(this.layer_id);
+		this.old_visible = layer.visible;
+		if (layer.visible == false)
+			layer.visible = true;
+		else
+			layer.visible = false;
+		app.Layers.render();
+		app.GUI.GUI_layers.render_layers();
+	}
+
+	async undo() {
+		super.undo();
+		const layer = app.Layers.get_layer(this.layer_id);
+		layer.visible = this.old_visible;
+		this.old_visible = null;
+		app.Layers.render();
+		app.GUI.GUI_layers.render_layers();
+	}
+}