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/u/[id]/UserProfileContent.tsx

@@ -0,0 +1,165 @@
+import Link from "next/link";
+import { FileText, MessageCircle, AlertCircle } from "lucide-react";
+import Pagination from "@/components/Pagination";
+import { IconComment, IconEye, IconHeart } from "@/components/Icons";
+import type { UserProfile, UserCommentsResponse } from "@/lib/api";
+
+/**
+ * 用户主页内容区（纯服务端渲染）：
+ * tab 与分页全部走 URL（?tab=comments&page=N），刷新/后退/分享链接均保持一致。
+ */
+export default function UserProfileContent({
+  userId,
+  profile,
+  tab,
+  commentData,
+  commentError,
+  page,
+}: {
+  userId: string;
+  profile: UserProfile;
+  tab: "posts" | "comments";
+  commentData: UserCommentsResponse | null;
+  commentError: boolean;
+  page: number;
+}) {
+  const tabLink = (t: "posts" | "comments", label: string, count: number, icon: React.ReactNode) => {
+    const active = tab === t;
+    const href = t === "posts" ? `/u/${userId}` : `/u/${userId}?tab=comments`;
+    return (
+      <Link
+        href={href}
+        aria-current={active ? "page" : undefined}
+        className={`inline-flex items-center justify-center gap-1.5 px-4 py-2 rounded-full text-sm font-semibold transition-all flex-1 sm:flex-none ${
+          active ? "" : "hover:text-[var(--ink)]"
+        }`}
+        style={{
+          background: active ? "var(--panel)" : "transparent",
+          color: active ? "var(--accent)" : "var(--ink-3)",
+          boxShadow: active ? "var(--shadow-sm)" : "none",
+        }}
+      >
+        {icon}
+        {label}
+        <span
+          className="text-[11px] font-bold px-1.5 py-px rounded-full tabular-nums"
+          style={{
+            background: active ? "var(--accent-soft)" : "var(--tint)",
+            color: active ? "var(--accent)" : "var(--ink-3)",
+          }}
+        >
+          {count}
+        </span>
+      </Link>
+    );
+  };
+
+  return (
+    <div>
+      <div
+        className="inline-flex w-full sm:w-auto items-center gap-1 mb-6 p-1 rounded-full"
+        style={{ background: "var(--panel-2)" }}
+      >
+        {tabLink("posts", "发帖", profile.stats.post_count, <FileText size={15} />)}
+        {tabLink("comments", "评论", profile.stats.comment_count, <MessageCircle size={15} />)}
+      </div>
+
+      {tab === "posts" ? (
+        <div>
+          {profile.posts.length > 0 ? (
+            <div className="panel overflow-hidden p-0">
+              {profile.posts.map((post, i) => (
+                <Link
+                  key={post.id}
+                  href={`/post/${post.id}`}
+                  className="group block px-4 sm:px-5 py-4 transition-colors hover:bg-[var(--panel-2)]"
+                  style={{ borderTop: i === 0 ? "none" : "1px solid var(--line)" }}
+                >
+                  <div className="flex items-center gap-2 mb-2 flex-wrap">
+                    <span className="j13-board-tag">{post.board.name}</span>
+                    {post.pinned > 0 && <span className="badge-pin">置顶</span>}
+                    {post.recommended && <span className="badge-rec">推荐</span>}
+                  </div>
+                  <h2
+                    className="font-semibold mb-2 leading-snug transition-colors group-hover:text-[var(--accent)]"
+                    style={{ color: "var(--ink)" }}
+                  >
+                    {post.title}
+                  </h2>
+                  <div className="flex items-center gap-4 meta text-[12px]">
+                    <span className="tabular-nums">{new Date(post.created_at).toLocaleDateString("zh-CN")}</span>
+                    <span className="inline-flex items-center gap-1">
+                      <IconComment size={12} /> {post.comment_count}
+                    </span>
+                    <span className="inline-flex items-center gap-1">
+                      <IconEye size={12} /> {post.view_count}
+                    </span>
+                    <span className="inline-flex items-center gap-1">
+                      <IconHeart size={12} /> {post.like_count}
+                    </span>
+                  </div>
+                </Link>
+              ))}
+            </div>
+          ) : (
+            <div className="panel text-center py-16 px-6">
+              <FileText size={26} className="mx-auto mb-3" style={{ color: "var(--ink-3)" }} />
+              <p className="text-sm font-semibold" style={{ color: "var(--ink-2)" }}>暂无发帖</p>
+            </div>
+          )}
+          <Pagination
+            total={profile.posts_total}
+            page={page}
+            size={20}
+            basePath={`/u/${userId}`}
+          />
+        </div>
+      ) : commentError ? (
+        <div className="panel text-center py-16 px-6" role="alert">
+          <AlertCircle size={26} className="mx-auto mb-3" style={{ color: "var(--danger)" }} />
+          <p className="text-sm font-semibold" style={{ color: "var(--ink-2)" }}>评论加载失败</p>
+          <p className="meta mt-1.5 mb-4">网络异常，请稍后重试</p>
+          <Link href={`/u/${userId}?tab=comments`} className="btn btn-line btn-sm">重新加载</Link>
+        </div>
+      ) : commentData && commentData.comments.length > 0 ? (
+        <div>
+          <div className="panel overflow-hidden p-0">
+            {commentData.comments.map((c, i) => (
+              <Link
+                key={c.id}
+                href={`/post/${c.post_id}`}
+                className="block px-4 sm:px-5 py-4 transition-colors hover:bg-[var(--panel-2)]"
+                style={{ borderTop: i === 0 ? "none" : "1px solid var(--line)" }}
+              >
+                <div className="meta text-xs mb-2">
+                  评论于《<span style={{ color: "var(--accent)" }}>{c.post_title}</span>》
+                </div>
+                <p
+                  className="text-[15px] leading-relaxed line-clamp-2"
+                  style={{ color: "var(--ink)" }}
+                >
+                  {c.content}
+                </p>
+                <div className="meta text-xs mt-2 tabular-nums">
+                  {new Date(c.created_at).toLocaleString("zh-CN")}
+                </div>
+              </Link>
+            ))}
+          </div>
+          <Pagination
+            total={commentData.total}
+            page={commentData.page}
+            size={20}
+            basePath={`/u/${userId}`}
+            query={{ tab: "comments" }}
+          />
+        </div>
+      ) : (
+        <div className="panel text-center py-16 px-6">
+          <MessageCircle size={26} className="mx-auto mb-3" style={{ color: "var(--ink-3)" }} />
+          <p className="text-sm font-semibold" style={{ color: "var(--ink-2)" }}>暂无评论</p>
+        </div>
+      )}
+    </div>
+  );
+}