feat: 公文三栏单页布局，并修复时间线目录跳转与样式

公告/单页对齐帖子详情三栏壳；时间线按日锚点可跳转，条目改为日志行并用暖金日期区分 feat 色。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/components/site-doc/SiteDocIdentity.tsx

@@ -0,0 +1,61 @@
+import Link from "next/link";
+import { Info, Megaphone, type LucideIcon } from "lucide-react";
+
+export type SiteDocIdentityKind = "announcements" | "about";
+
+const META: Record<
+  SiteDocIdentityKind,
+  { title: string; href: string; Icon: LucideIcon; fallbackLead: string }
+> = {
+  announcements: {
+    title: "站点公告",
+    href: "/announcements",
+    Icon: Megaphone,
+    fallbackLead: "官方通知与运营公告",
+  },
+  about: {
+    title: "关于本站",
+    href: "/",
+    Icon: Info,
+    fallbackLead: "站点说明与政策文档",
+  },
+};
+
+/** 左栏身份卡：标明当前公文区 */
+export default function SiteDocIdentity({
+  kind,
+  lead,
+}: {
+  kind: SiteDocIdentityKind;
+  lead?: string;
+}) {
+  const m = META[kind];
+  const Icon = m.Icon;
+  const text = lead?.trim() || m.fallbackLead;
+
+  return (
+    <section className="j13-module shrink-0" aria-labelledby={`site-doc-id-${kind}`}>
+      <p className="j13-module-title mb-3" id={`site-doc-id-${kind}`}>
+        <Icon size={15} style={{ color: "var(--accent)" }} /> {m.title}
+      </p>
+      <Link href={m.href} className="flex items-center gap-3 group">
+        <span
+          className="w-11 h-11 rounded-xl flex items-center justify-center shrink-0"
+          style={{ background: "var(--accent-soft)", color: "var(--accent)" }}
+          aria-hidden
+        >
+          <Icon size={20} strokeWidth={2} />
+        </span>
+        <span
+          className="font-semibold text-[14.5px] truncate group-hover:text-[var(--accent)] transition-colors"
+          style={{ color: "var(--ink)" }}
+        >
+          {m.title}
+        </span>
+      </Link>
+      <p className="mt-2.5 text-[12.5px] leading-relaxed" style={{ color: "var(--ink-3)" }}>
+        {text}
+      </p>
+    </section>
+  );
+}