[CL-881] restore nav tree view (#17210)

* restore tree nav view

* address Claude feedback

* address feedback and fix depth calculation

* address feedback from Claude

* do not reserve space for icon

libs/components/src/navigation/nav-base.component.ts

@@ -1,4 +1,4 @@
-import { Directive, EventEmitter, Output, input } from "@angular/core";
+import { Directive, EventEmitter, Output, input, model } from "@angular/core";
 import { RouterLink, RouterLinkActive } from "@angular/router";
 
 /**
@@ -21,6 +21,16 @@ export abstract class NavBaseComponent {
    */
   readonly icon = input<string>();
 
+  /**
+   * If this item is used within a tree, set `variant` to `"tree"`
+   */
+  readonly variant = input<"default" | "tree">("default");
+
+  /**
+   * Depth level when nested inside of a `'tree'` variant
+   */
+  readonly treeDepth = model(0);
+
   /**
    * Optional route to be passed to internal `routerLink`. If not provided, the nav component will render as a button.
    *