feat(ui): toggle the diagram-list sidebar

Add a ☰ toolbar button (and ⌘/Ctrl+B shortcut) to show/hide the left sidebar
for a wider editing/preview area. The preview re-fits when the sidebar — or the
Optimize panel — toggles, so the diagram stays centred.

src/App.svelte

@@ -34,6 +34,11 @@
       e.preventDefault();
       void store.save();
     }
+    // ⌘/Ctrl+B: toggle the diagram-list sidebar.
+    if ((e.metaKey || e.ctrlKey) && e.key.toLowerCase() === 'b') {
+      e.preventDefault();
+      store.showSidebar = !store.showSidebar;
+    }
     // ⌘/Ctrl+E: jump into viewer mode and toggle the quick-edit drawer.
     if ((e.metaKey || e.ctrlKey) && e.key.toLowerCase() === 'e') {
       e.preventDefault();
@@ -54,7 +59,9 @@
   <div class="app">
     <Toolbar />
     <div class="body">
-      <Sidebar />
+      {#if store.showSidebar}
+        <Sidebar />
+      {/if}
       {#if store.activeId}
         <div class="split" bind:this={splitHost} class:splitting>
           {#if store.layout !== 'preview'}