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/announcement/[id]/page.tsx

@@ -0,0 +1,99 @@
+import type { Metadata } from "next";
+import Link from "next/link";
+import { notFound } from "next/navigation";
+import ReactMarkdown from "react-markdown";
+import { ChevronRight, Megaphone } from "lucide-react";
+import { fetchAnnouncementDetail } from "@/lib/api";
+import { formatDate } from "@/lib/format";
+
+interface PageProps {
+  params: Promise<{ id: string }>;
+}
+
+export async function generateMetadata({ params }: PageProps): Promise<Metadata> {
+  const { id } = await params;
+  const data = await fetchAnnouncementDetail(id).catch(() => null);
+  if (!data) return { title: "公告不存在" };
+  return {
+    title: data.announcement.title,
+    description: `站点公告 · ${data.announcement.tag}`,
+    openGraph: {
+      title: data.announcement.title,
+      description: `站点公告 · ${data.announcement.tag}`,
+      type: "article",
+      publishedTime: data.announcement.created_at,
+    },
+  };
+}
+
+export default async function AnnouncementPage({ params }: PageProps) {
+  const { id } = await params;
+  const data = await fetchAnnouncementDetail(id).catch(() => null);
+  if (!data) notFound();
+
+  const a = data.announcement;
+  const jsonLd = {
+    "@context": "https://schema.org",
+    "@type": "Article",
+    headline: a.title,
+    datePublished: a.created_at,
+    dateModified: a.updated_at,
+    articleSection: "站点公告",
+    keywords: a.tag,
+  };
+
+  return (
+    <div className="max-w-3xl mx-auto">
+      <script
+        type="application/ld+json"
+        dangerouslySetInnerHTML={{ __html: JSON.stringify(jsonLd) }}
+      />
+
+      {/* 面包屑 */}
+      <nav className="meta mb-5 flex items-center gap-1.5 text-[13px]">
+        <Link href="/" className="hover:text-[var(--accent)] transition-colors">
+          首页
+        </Link>
+        <ChevronRight size={13} />
+        <span style={{ color: "var(--accent)" }}>站点公告</span>
+      </nav>
+
+      <article className="panel p-6 sm:p-8">
+        <div className="flex items-center justify-between gap-3 flex-wrap">
+          <span className={`j13-anno-tag is-${a.tag_color}`}>{a.tag}</span>
+          <time className="meta text-[12px] tabular-nums" dateTime={a.created_at}>
+            {formatDate(a.created_at)}
+          </time>
+        </div>
+
+        <h1
+          className="mt-3.5 text-[24px] sm:text-[28px] font-extrabold leading-snug tracking-tight"
+          style={{ color: "var(--ink)" }}
+        >
+          {a.title}
+        </h1>
+
+        <div
+          className="mt-5 pt-5"
+          style={{ borderTop: "1px solid var(--line)" }}
+        >
+          <div className="prose-content">
+            <ReactMarkdown>{a.content}</ReactMarkdown>
+          </div>
+        </div>
+      </article>
+
+      <div className="mt-5 flex items-center gap-2 text-[12.5px] meta">
+        <Megaphone size={14} style={{ color: "var(--accent)" }} />
+        <span>由站点管理团队发布</span>
+        <Link
+          href="/"
+          className="ml-auto font-semibold transition-colors hover:text-[var(--accent)]"
+          style={{ color: "var(--accent)" }}
+        >
+          返回首页
+        </Link>
+      </div>
+    </div>
+  );
+}