feat: 新增排行榜模块

- 新增 leaderboard 服务与 handler、前端排行榜页面
- 路由与首页入口适配

frontend/components/Header.tsx

@@ -14,6 +14,7 @@ import {
   Settings,
   Bell,
   Home,
+  Trophy,
   User as UserIcon,
   ChevronDown,
   ShieldCheck,
@@ -241,6 +242,13 @@ export default function Header({
             <NavPill href="/" active={pathname === "/"} icon={<Home size={15} />}>
               首页
             </NavPill>
+            <NavPill
+              href="/leaderboard"
+              active={pathname?.startsWith("/leaderboard") ?? false}
+              icon={<Trophy size={15} />}
+            >
+              排行榜
+            </NavPill>
           </nav>
         </div>
 
@@ -366,6 +374,11 @@ export default function Header({
           </form>
           <div className="flex flex-col">
             <MobileLink href="/" active={pathname === "/"}>首页</MobileLink>
+            <MobileLink href="/leaderboard" active={pathname?.startsWith("/leaderboard") ?? false}>
+              <span className="inline-flex items-center gap-2">
+                <Trophy size={15} /> 排行榜
+              </span>
+            </MobileLink>
             {user && !hideComposeNav && <MobileLink href="/compose">发帖</MobileLink>}
             {user && allowMessages && (
               <MobileLink href="/chat" active={pathname?.startsWith("/chat") ?? false}>