feat: 新增评论框提示文案自定义功能并优化侧边栏排序

- 新增评论框提示文案（placeholder）自定义功能，支持在后台设置
- 更新文档：重命名“点赞开关”为“功能开关”，调整侧边栏排序
- 优化前端组件，统一处理评论表单和回复编辑器的提示文案
- 修复CSS样式，确保操作按钮始终可见

cwd-api/src/api/admin/featureSettings.ts

@@ -25,10 +25,15 @@ export const updateFeatureSettings = async (c: Context<{ Bindings: Bindings }>)
 			typeof body.enableArticleLike === 'boolean'
 				? body.enableArticleLike
 				: undefined;
+		const rawCommentPlaceholder =
+			typeof body.commentPlaceholder === 'string' ? body.commentPlaceholder : undefined;
+		const commentPlaceholder =
+			rawCommentPlaceholder !== undefined ? rawCommentPlaceholder.trim() : undefined;
 
 		await saveFeatureSettings(c.env, {
 			enableCommentLike,
-			enableArticleLike
+			enableArticleLike,
+			commentPlaceholder
 		});
 
 		return c.json({ message: '保存成功！' });