feat: 完成社区首页功能迭代，新增首页聚合与工作台组件

本次提交实现了完整的社区首页功能：
1. 新增首页聚合接口与服务，包含社区统计、热门帖、活跃用户与板块数据
2. 实现帖子推荐排序、SSR缓存与点赞态同步逻辑
3. 重构导航与布局系统，新增工作台侧边栏与移动端适配
4. 优化主题色与UI样式，统一设计系统风格
5. 修复登录注册页自动跳转与组件状态同步问题

frontend/components/home/FeedTabs.tsx

@@ -0,0 +1,57 @@
+"use client";
+
+import Link from "next/link";
+import { useRouter } from "next/navigation";
+import { useRef } from "react";
+
+export type FeedSort = "latest" | "hot" | "recommended";
+
+const TAB_KEYS: FeedSort[] = ["latest", "hot", "recommended"];
+const TAB_LABELS: Record<FeedSort, string> = {
+  latest: "最新",
+  hot: "热门",
+  recommended: "精华",
+};
+
+// 在指定板块内切换排序时保留 ?board=
+function tabHref(key: FeedSort, boardId: number) {
+  const params = new URLSearchParams();
+  if (boardId > 0) params.set("board", String(boardId));
+  if (key !== "latest") params.set("sort", key);
+  const qs = params.toString();
+  return qs ? `/?${qs}` : "/";
+}
+
+// 中栏排序分段控件：URL query 驱动、SSR 直出，键盘 ←/→ 可切换
+export default function FeedTabs({ current, boardId = 0 }: { current: FeedSort; boardId?: number }) {
+  const router = useRouter();
+  const tabRefs = useRef<(HTMLAnchorElement | null)[]>([]);
+
+  const handleKeyDown = (e: React.KeyboardEvent, i: number) => {
+    if (e.key !== "ArrowRight" && e.key !== "ArrowLeft") return;
+    e.preventDefault();
+    const next = (i + (e.key === "ArrowRight" ? 1 : TAB_KEYS.length - 1)) % TAB_KEYS.length;
+    tabRefs.current[next]?.focus();
+    router.push(tabHref(TAB_KEYS[next], boardId));
+  };
+
+  return (
+    <div className="j13-seg" role="tablist" aria-label="帖子排序">
+      {TAB_KEYS.map((key, i) => (
+        <Link
+          key={key}
+          ref={(el) => {
+            tabRefs.current[i] = el;
+          }}
+          href={tabHref(key, boardId)}
+          role="tab"
+          aria-selected={current === key}
+          tabIndex={current === key ? 0 : -1}
+          onKeyDown={(e) => handleKeyDown(e, i)}
+        >
+          {TAB_LABELS[key]}
+        </Link>
+      ))}
+    </div>
+  );
+}