add app layout

src/App.svelte

@@ -1,5 +1,19 @@
 <script>
+  import Layout from './lib/Layout.svelte';
   import WorldMap from './lib/WorldMap.svelte';
+  import Timeline from './lib/Timeline.svelte';
+
+  let screen = $state('worldmap');
+
+  function onNavigate(s) {
+    screen = s;
+  }
 </script>
 
-<WorldMap />
+<Layout {screen} {onNavigate}>
+  {#if screen === 'worldmap'}
+    <WorldMap />
+  {:else}
+    <Timeline />
+  {/if}
+</Layout>