Use basic sidebar composition for post comments

src/css/compositions/sidebar.css

@@ -0,0 +1,10 @@
+.sidebar {
+	display: flex;
+	gap: var(--vertical-spacing, 1rem);
+	flex-wrap: wrap;
+
+	> * {
+		flex-grow: 1;
+		flex-shrink: 1;
+	}
+}