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/about.gjs

@@ -0,0 +1,63 @@
+import { on } from '@ember/modifier';
+import Icon from '#components/icon';
+
+<template>
+  <div class="sidebar-header">
+    <button type="button" class="back-btn" {{on "click" @onBack}}>
+      <Icon @name="arrow-left" @size={{20}} @color="#333" />
+    </button>
+    <h2>About</h2>
+    <button type="button" class="close-btn" {{on "click" @onClose}}>
+      <Icon @name="x" @size={{20}} @color="#333" />
+    </button>
+  </div>
+
+  <div class="sidebar-content">
+    <section class="settings-section">
+      <p>
+        <strong>Marco</strong>
+        (as in
+        <a
+          href="https://en.wikipedia.org/wiki/Marco_Polo"
+          target="_blank"
+          rel="noopener"
+        >Marco Polo</a>) is an unhosted maps application that respects your
+        privacy and choices.
+      </p>
+      <p>
+        Connect your own
+        <a
+          href="https://remotestorage.io/"
+          target="_blank"
+          rel="noopener"
+        >remote storage</a>
+        to sync place bookmarks across apps and devices.
+      </p>
+      <ul class="link-list">
+        <li>
+          <a
+            href="https://gitea.kosmos.org/raucao/marco"
+            target="_blank"
+            rel="noopener"
+          >
+            Source Code
+          </a>
+          (<a
+            href="https://en.wikipedia.org/wiki/GNU_Affero_General_Public_License"
+            target="_blank"
+            rel="noopener"
+          >AGPL</a>)
+        </li>
+        <li>
+          <a
+            href="https://openstreetmap.org/copyright"
+            target="_blank"
+            rel="noopener"
+          >
+            Map Data © OpenStreetMap
+          </a>
+        </li>
+      </ul>
+    </section>
+  </div>
+</template>