feat(home): add homepage hero banner and migrate community stats

- 新增首页欢迎横幅组件，包含时段问候、用户问候语、发帖引导按钮和社区统计数据
- 从CommunityPanels中移除stats相关逻辑，将社区统计迁移到新的HomeHero组件中
- 优化全局样式，添加横幅相关的CSS动画和样式类

frontend/app/page.tsx

@@ -14,6 +14,7 @@ import PostRow from "@/components/PostRow";
 import FeedTabs, { type FeedSort } from "@/components/home/FeedTabs";
 import { WorkspaceNav, WorkspaceNavMobile } from "@/components/home/WorkspaceNav";
 import { CommunityPanels, CommunityPanelsMobile } from "@/components/home/CommunityPanels";
+import HomeHero from "@/components/home/HomeHero";
 
 export const metadata: Metadata = {
   title: "姜十三论坛 · 技术讨论社区",
@@ -150,6 +151,8 @@ export default async function HomePage({ searchParams }: HomePageProps) {
 
   return (
     <div>
+      <HomeHero user={user} stats={overview?.stats ?? null} />
+
       <WorkspaceNavMobile boards={boards} user={user} />
 
       <div className="flex items-start gap-6 xl:gap-7 mt-5 lg:mt-0">
@@ -201,22 +204,14 @@ export default async function HomePage({ searchParams }: HomePageProps) {
 
         {/* 右栏：社区面板 */}
         {overview && (
-          <CommunityPanels
-            stats={overview.stats}
-            hot={overview.hot}
-            activeUsers={overview.active_users}
-          />
+          <CommunityPanels hot={overview.hot} activeUsers={overview.active_users} />
         )}
       </div>
 
       {/* 窄屏：面板下沉到帖子流之后 */}
       {overview && (
         <div className="mt-6">
-          <CommunityPanelsMobile
-            stats={overview.stats}
-            hot={overview.hot}
-            activeUsers={overview.active_users}
-          />
+          <CommunityPanelsMobile hot={overview.hot} activeUsers={overview.active_users} />
         </div>
       )}
     </div>