Refactor app menu sidebar

* Rename to "app menu" across code
* Move content to dedicated sections/components, introduce app menu links
* Use CSS variable for hover background color across the app

app/components/app-menu/index.gjs

@@ -0,0 +1,38 @@
+import Component from '@glimmer/component';
+import { action } from '@ember/object';
+import { tracked } from '@glimmer/tracking';
+import { fn } from '@ember/helper';
+import eq from 'ember-truth-helpers/helpers/eq';
+
+import AppMenuHome from './home';
+import AppMenuSettings from './settings';
+import AppMenuAbout from './about';
+
+export default class AppMenu extends Component {
+  @tracked currentView = 'menu'; // 'menu', 'settings', 'about'
+
+  @action
+  setView(view) {
+    this.currentView = view;
+  }
+
+  <template>
+    <div class="sidebar settings-pane">
+      {{#if (eq this.currentView "menu")}}
+        <AppMenuHome @onNavigate={{this.setView}} @onClose={{@onClose}} />
+
+      {{else if (eq this.currentView "settings")}}
+        <AppMenuSettings
+          @onBack={{fn this.setView "menu"}}
+          @onClose={{@onClose}}
+        />
+
+      {{else if (eq this.currentView "about")}}
+        <AppMenuAbout
+          @onBack={{fn this.setView "menu"}}
+          @onClose={{@onClose}}
+        />
+      {{/if}}
+    </div>
+  </template>
+}