Default UI redesign (#7429)

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* Update default.sidebar.vue

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* Update sticky-sidebar.ts

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* Update messaging-room.form.vue

* Update timeline.vue

src/client/ui/default.side.vue

@@ -118,7 +118,7 @@ export default defineComponent({
 .qvzfzxam {
 	$header-height: 58px; // TODO: どこかに集約したい
 
-	--section-padding: 16px;
+	--root-margin: 16px;
 	--margin: var(--marginHalf);
 
 	> .container {