Add full-text search

Add a search box with a quick results popover, as well full results in
the sidebar on pressing enter.

tests/integration/components/app-header-test.gjs

@@ -0,0 +1,19 @@
+import { module, test } from 'qunit';
+import { setupRenderingTest } from 'marco/tests/helpers';
+import { render } from '@ember/test-helpers';
+import AppHeader from 'marco/components/app-header';
+
+module('Integration | Component | app-header', function (hooks) {
+  setupRenderingTest(hooks);
+
+  test('it renders the search box', async function (assert) {
+    this.noop = () => {};
+    await render(
+      <template><AppHeader @onToggleMenu={{this.noop}} /></template>
+    );
+
+    assert.dom('header.app-header').exists();
+    assert.dom('.search-box').exists('Search box is present in the header');
+    assert.dom('.menu-btn').exists('Menu button is present');
+  });
+});