feat: 新增站点公告系统与多项体验优化

此提交完成了以下核心功能与优化：
1. 新增站点公告全流程支持：包含公告模型、CRUD接口、管理后台页面、公告详情页与首页展示
2. 实现评论楼层化与回复嵌套：重构评论系统为树状结构，支持回复评论与层级限制
3. 添加用户在线状态追踪：异步更新用户最后活跃时间并做SQL限频
4. 优化用户体验：移动端浮动操作条、统一板块标签样式、用户页签到积分展示、注册即登录优化
5. 其他细节优化：SEO站点地图新增公告页、评论分页与统计、登录/注册表单增强等

frontend/app/notifications/NotificationsClient.tsx

@@ -0,0 +1,124 @@
+"use client";
+
+import { useState } from "react";
+import { useRouter } from "next/navigation";
+import Link from "next/link";
+import { Heart, MessageCircle, CheckCheck, Bell } from "lucide-react";
+import { apiMarkRead, apiMarkAllRead, type NotificationsResponse, type NotificationItem } from "@/lib/api";
+
+/**
+ * 通知列表交互层：数据由服务端 SSR 直出，这里只承担已读标记等客户端操作。
+ */
+export default function NotificationsClient({ initialData }: { initialData: NotificationsResponse }) {
+  const router = useRouter();
+  const [notifications, setNotifications] = useState<NotificationItem[]>(initialData.notifications || []);
+
+  const handleClick = async (n: NotificationItem) => {
+    if (!n.is_read) {
+      apiMarkRead(n.id).catch(() => {});
+      setNotifications((list) =>
+        list.map((item) => (item.id === n.id ? { ...item, is_read: true } : item))
+      );
+    }
+    router.push(`/post/${n.post_id}`);
+  };
+
+  const handleMarkAll = async () => {
+    await apiMarkAllRead();
+    setNotifications((list) => list.map((item) => ({ ...item, is_read: true })));
+    // 同步刷新 Header 未读红点（layout 重新取 unread_count）
+    router.refresh();
+  };
+
+  const formatTime = (dateStr: string) => {
+    const diff = Date.now() - new Date(dateStr).getTime();
+    const min = Math.floor(diff / 60000);
+    if (min < 1) return "刚刚";
+    if (min < 60) return `${min} 分钟前`;
+    const hour = Math.floor(min / 60);
+    if (hour < 24) return `${hour} 小时前`;
+    return `${Math.floor(hour / 24)} 天前`;
+  };
+
+  const hasUnread = notifications.some((n) => !n.is_read);
+
+  return (
+    <>
+      {hasUnread && (
+        <div className="flex justify-end mb-4">
+          <button onClick={handleMarkAll} className="btn btn-line btn-sm">
+            <CheckCheck size={14} /> 全部已读
+          </button>
+        </div>
+      )}
+
+      {notifications.length === 0 ? (
+        <div className="panel text-center py-20 px-6">
+          <span
+            className="inline-flex w-14 h-14 rounded-full items-center justify-center mb-4"
+            style={{ background: "var(--accent-soft)", color: "var(--accent)" }}
+          >
+            <Bell size={26} />
+          </span>
+          <p className="font-bold text-lg" style={{ color: "var(--ink)" }}>还没有通知</p>
+          <p className="meta mt-1.5">当有人评论或点赞你的帖子时，会在这里提醒你</p>
+        </div>
+      ) : (
+        <div className="panel overflow-hidden p-0">
+          {notifications.map((n, i) => (
+            <button
+              key={n.id}
+              onClick={() => handleClick(n)}
+              className="w-full text-left px-4 sm:px-5 py-4 flex items-start gap-3.5 transition-colors hover:bg-[var(--panel-2)]"
+              style={{
+                borderTop: i === 0 ? "none" : "1px solid var(--line)",
+                background: n.is_read ? "transparent" : "var(--panel-2)",
+              }}
+            >
+              <span
+                className="w-10 h-10 shrink-0 rounded-full flex items-center justify-center"
+                style={{
+                  color: n.type === "like" ? "var(--clay)" : "var(--accent)",
+                  background:
+                    n.type === "like" ? "var(--clay-soft)" : "var(--accent-soft)",
+                }}
+              >
+                {n.type === "like" ? <Heart size={16} /> : <MessageCircle size={16} />}
+              </span>
+              <div className="flex-1 min-w-0">
+                <p className="text-sm leading-snug" style={{ color: "var(--ink)" }}>
+                  <Link
+                    href={`/u/${n.actor.id}`}
+                    onClick={(e) => e.stopPropagation()}
+                    className="font-semibold hover:text-[var(--accent)] transition-colors"
+                  >
+                    {n.actor.nickname}
+                  </Link>{" "}
+                  <span className="meta">
+                    {n.type === "comment"
+                      ? "评论了你的帖子"
+                      : n.type === "reply"
+                        ? "回复了你的评论"
+                        : "点赞了你的帖子"}
+                  </span>
+                </p>
+                <p className="text-sm mt-1 truncate" style={{ color: "var(--ink-2)" }}>
+                  《{n.post.title}》
+                </p>
+                {n.content && (
+                  <p className="text-[13px] mt-1.5 line-clamp-2" style={{ color: "var(--ink-3)" }}>
+                    {n.content}
+                  </p>
+                )}
+                <p className="meta text-xs mt-2">{formatTime(n.created_at)}</p>
+              </div>
+              {!n.is_read && (
+                <span className="w-2.5 h-2.5 shrink-0 mt-2 rounded-full" style={{ background: "var(--clay)" }} />
+              )}
+            </button>
+          ))}
+        </div>
+      )}
+    </>
+  );
+}