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/SiteDocAnnoNav.tsx

@@ -0,0 +1,82 @@
+import Link from "next/link";
+import { Megaphone, Pin } from "lucide-react";
+import type { AnnouncementSummary } from "@/lib/api";
+import { formatShortDate } from "@/lib/format";
+
+/** 近期公告目录（高亮当前 id） */
+export default function SiteDocAnnoNav({
+  items,
+  currentId,
+  showAllLink = true,
+}: {
+  items: AnnouncementSummary[];
+  currentId?: number;
+  showAllLink?: boolean;
+}) {
+  return (
+    <section className="j13-module flex min-h-0 flex-col" aria-label="近期公告">
+      <div className="flex items-center justify-between gap-2 shrink-0">
+        <p className="j13-module-title !mb-0">
+          <Megaphone size={15} style={{ color: "var(--accent)" }} /> 近期公告
+        </p>
+        {showAllLink ? (
+          <Link
+            href="/announcements"
+            className="meta text-[12px] font-normal transition-colors hover:text-[var(--accent)]"
+          >
+            全部
+          </Link>
+        ) : null}
+      </div>
+      {items.length === 0 ? (
+        <p className="meta mt-2 px-0.5">暂无公告</p>
+      ) : (
+        <ul className="j13-anno-list j13-rail-scroll">
+          {items.map((a) => {
+            const current = currentId === a.id;
+            return (
+              <li key={a.id}>
+                <Link
+                  href={`/announcement/${a.id}`}
+                  className="j13-anno-item"
+                  data-tip={a.title}
+                  aria-current={current ? "page" : undefined}
+                  style={
+                    current
+                      ? {
+                          background: "var(--accent-soft)",
+                          borderColor: "color-mix(in srgb, var(--accent) 28%, var(--line))",
+                        }
+                      : undefined
+                  }
+                >
+                  <span className="j13-anno-item-body">
+                    <span className="j13-anno-item-title truncate inline-flex items-center gap-1 min-w-0">
+                      {a.pinned ? (
+                        <Pin
+                          size={11}
+                          className="shrink-0"
+                          style={{ color: "var(--accent)" }}
+                          aria-label="置顶"
+                        />
+                      ) : null}
+                      <span className="truncate">{a.title}</span>
+                    </span>
+                    <span className="j13-anno-item-meta">
+                      {a.tag ? (
+                        <span className={`j13-anno-tag is-${a.tag_color}`}>{a.tag}</span>
+                      ) : null}
+                      <span className="meta text-[11px] tabular-nums">
+                        {formatShortDate(a.created_at)}
+                      </span>
+                    </span>
+                  </span>
+                </Link>
+              </li>
+            );
+          })}
+        </ul>
+      )}
+    </section>
+  );
+}