feat: 增强评论功能，添加评论未更改提示，优化评论输入组件的占位符

web/src/locales/zh-CN.json

@@ -7,6 +7,7 @@
     "comment": "评论",
     "comment_failed": "评论失败",
     "comment_success": "评论成功！",
+    "comment_unchanged": "评论内容未更改。",
     "confirm_delete": "确定吗？",
     "content_required": "评论内容不能为空。",
     "delete": "删除",
@@ -20,8 +21,9 @@
     "like_failed": "点赞失败",
     "like_success": "点赞成功",
     "login_required": "请先登录后再操作",
-    "placeholder": "写你的评论...",
+    "placeholder": "写下你的评论...",
     "private": "私密评论",
+    "private_placeholder": "悄悄地说一句...",
     "reply": "回复",
     "submit": "提交",
     "unlike": "取消点赞",