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/WorkspaceNav.tsx

@@ -0,0 +1,122 @@
+"use client";
+
+import Link from "next/link";
+import { useSearchParams } from "next/navigation";
+import { PenSquare, Home, Bell, Settings, LogIn, UserPlus } from "lucide-react";
+import type { BoardWithCount, User } from "@/lib/api";
+
+// 首页壳内板块切换：仅替换中栏帖子流，不离开工作台
+function boardHref(id: number) {
+  return `/?board=${id}`;
+}
+
+// 桌面左栏：发帖 CTA + 板块导航 + 登录态快捷入口
+export function WorkspaceNav({ boards, user }: { boards: BoardWithCount[]; user: User | null }) {
+  const params = useSearchParams();
+  const activeBoard = Number(params.get("board")) || 0;
+  const composeHref = user ? "/compose" : "/login?redirect=/compose";
+
+  return (
+    <nav className="hidden lg:block w-56 xl:w-60 shrink-0" aria-label="工作台导航">
+      <div className="sticky top-24 space-y-5">
+        <Link href={composeHref} className="btn btn-primary w-full justify-center">
+          <PenSquare size={16} /> 发起讨论
+        </Link>
+
+        <div>
+          <p className="px-2.5 mb-1.5 text-[11.5px] font-bold tracking-[0.08em] uppercase" style={{ color: "var(--ink-3)" }}>
+            板块
+          </p>
+          <div className="space-y-0.5">
+            <Link
+              href="/"
+              className="j13-raillink"
+              aria-current={activeBoard === 0 ? "page" : undefined}
+            >
+              <Home size={16} className="shrink-0" />
+              <span className="flex-1 truncate">全部讨论</span>
+            </Link>
+            {boards.map((b, i) => (
+              <Link
+                key={b.id}
+                href={boardHref(b.id)}
+                className="j13-raillink"
+                aria-current={activeBoard === b.id ? "page" : undefined}
+              >
+                <span
+                  className={`av-${i % 8} w-7 h-7 rounded-lg flex items-center justify-center text-[12px] font-bold shrink-0`}
+                >
+                  {b.name.slice(0, 1)}
+                </span>
+                <span className="flex-1 truncate">{b.name}</span>
+                <span className="meta text-[11.5px]">{b.post_count}</span>
+              </Link>
+            ))}
+          </div>
+        </div>
+
+        <div style={{ borderTop: "1px solid var(--line)" }} className="pt-3 space-y-0.5">
+          {user ? (
+            <>
+              <Link href="/notifications" className="j13-raillink">
+                <Bell size={16} className="shrink-0" /> 通知中心
+              </Link>
+              <Link href="/settings" className="j13-raillink">
+                <Settings size={16} className="shrink-0" /> 账号设置
+              </Link>
+            </>
+          ) : (
+            <>
+              <Link href="/login" className="j13-raillink">
+                <LogIn size={16} className="shrink-0" /> 登录
+              </Link>
+              <Link href="/register" className="j13-raillink">
+                <UserPlus size={16} className="shrink-0" /> 注册账号
+              </Link>
+            </>
+          )}
+        </div>
+      </div>
+    </nav>
+  );
+}
+
+// 移动端：发帖入口 + 横向滑动板块条
+export function WorkspaceNavMobile({
+  boards,
+  user,
+}: {
+  boards: BoardWithCount[];
+  user: User | null;
+}) {
+  const params = useSearchParams();
+  const activeBoard = Number(params.get("board")) || 0;
+  const composeHref = user ? "/compose" : "/login?redirect=/compose";
+
+  return (
+    <div className="lg:hidden -mx-4 sm:-mx-6 px-4 sm:px-6">
+      <div className="j13-chips">
+        <Link href={composeHref} className="btn btn-primary btn-sm shrink-0">
+          <PenSquare size={14} /> 发帖
+        </Link>
+        <Link
+          href="/"
+          className="j13-chip"
+          aria-current={activeBoard === 0 ? "page" : undefined}
+        >
+          <Home size={13} /> 全部
+        </Link>
+        {boards.map((b) => (
+          <Link
+            key={b.id}
+            href={boardHref(b.id)}
+            className="j13-chip"
+            aria-current={activeBoard === b.id ? "page" : undefined}
+          >
+            {b.name}
+          </Link>
+        ))}
+      </div>
+    </div>
+  );
+}