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/SiteDocMasthead.tsx

@@ -0,0 +1,94 @@
+import type { ReactNode } from "react";
+import { Clock } from "lucide-react";
+import { formatDate } from "@/lib/format";
+
+function formatDateTime(dateStr: string) {
+  return new Date(dateStr).toLocaleString("zh-CN", {
+    year: "numeric",
+    month: "2-digit",
+    day: "2-digit",
+    hour: "2-digit",
+    minute: "2-digit",
+  });
+}
+
+/** 公文头：kicker / 标签 / 大标题 / 导语 / 元信息 */
+export default function SiteDocMasthead({
+  kicker,
+  title,
+  excerpt,
+  createdAt,
+  updatedAt,
+  readingMinutes,
+  badges,
+  showTimeOfDay = false,
+}: {
+  kicker: string;
+  title: string;
+  excerpt?: string;
+  createdAt: string;
+  updatedAt?: string;
+  readingMinutes?: number;
+  badges?: ReactNode;
+  /** 详情页用完整日期时间；列表头图可用短日期 */
+  showTimeOfDay?: boolean;
+}) {
+  const edited =
+    updatedAt &&
+    new Date(updatedAt).getTime() - new Date(createdAt).getTime() > 60_000;
+  const publishedLabel = showTimeOfDay ? formatDateTime(createdAt) : formatDate(createdAt);
+  const updatedLabel =
+    edited && updatedAt
+      ? showTimeOfDay
+        ? formatDateTime(updatedAt)
+        : formatDate(updatedAt)
+      : null;
+
+  return (
+    <header className="mb-6 sm:mb-7">
+      <div className="flex items-center gap-2 flex-wrap mb-3">
+        <span
+          className="inline-flex items-center rounded-full px-2.5 py-0.5 text-[11.5px] font-semibold tracking-wide"
+          style={{ background: "var(--accent-soft)", color: "var(--accent)" }}
+        >
+          {kicker}
+        </span>
+        {badges}
+      </div>
+
+      <h1
+        className="text-[26px] sm:text-[34px] font-extrabold leading-tight tracking-tight"
+        style={{ color: "var(--ink)" }}
+      >
+        {title}
+      </h1>
+
+      {excerpt ? (
+        <p
+          className="mt-3.5 text-[15px] sm:text-[15.5px] leading-relaxed"
+          style={{ color: "var(--ink-2)" }}
+        >
+          {excerpt}
+        </p>
+      ) : null}
+
+      <div
+        className="mt-5 pt-4 flex flex-wrap items-center gap-x-4 gap-y-2"
+        style={{ borderTop: "1px solid var(--line)" }}
+      >
+        <p className="meta text-[12.5px]">
+          发布于 {publishedLabel}
+          {updatedLabel ? <> · 更新于 {updatedLabel}</> : null}
+        </p>
+        {readingMinutes != null ? (
+          <span
+            className="meta inline-flex items-center gap-1.5 text-[12.5px] sm:ml-auto"
+            aria-label={`阅读约需 ${readingMinutes} 分钟`}
+          >
+            <Clock size={13} /> 约 {readingMinutes} 分钟读完
+          </span>
+        ) : null}
+      </div>
+    </header>
+  );
+}