feat(layout): finalize p01 and p02 changes

Complete UI foundation and app layout implementation, stabilize container health checks, and archive both OpenSpec changes after verification.

frontend/tests/component/sidebar-item.test.ts

@@ -0,0 +1,9 @@
+import { describe, expect, it } from 'vitest';
+import SidebarItem from '../../src/lib/components/layout/SidebarItem.svelte';
+
+describe('SidebarItem component', () => {
+  it('exports a component module', () => {
+    expect(SidebarItem).toBeDefined();
+    expect(typeof SidebarItem).toBe('function');
+  });
+});