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/components/home/HomeHero.tsx

@@ -0,0 +1,76 @@
+import Link from "next/link";
+import { PenSquare, LogIn, UserPlus, MessagesSquare, Users, MessageCircle, Sparkles } from "lucide-react";
+import type { OverviewStats } from "@/lib/api";
+
+type HeroUser = { username: string; nickname: string } | null;
+
+function greetingOf(h: number): string {
+  if (h >= 5 && h < 11) return "早上好";
+  if (h >= 11 && h < 14) return "中午好";
+  if (h >= 14 && h < 18) return "下午好";
+  if (h >= 18 && h < 23) return "晚上好";
+  return "夜深了";
+}
+
+// 首页欢迎横幅：时段问候 + 站点主张 + 发帖引导 + 内联社区数据。纯服务端组件。
+export default function HomeHero({ user, stats }: { user: HeroUser; stats: OverviewStats | null }) {
+  const name = user ? user.nickname || user.username : null;
+  const hello = greetingOf(new Date().getHours());
+  const today = stats?.today_posts ?? 0;
+
+  const statItems = stats
+    ? [
+        { icon: MessagesSquare, label: "讨论", value: stats.posts, hot: false },
+        { icon: Users, label: "成员", value: stats.users, hot: false },
+        { icon: MessageCircle, label: "评论", value: stats.comments, hot: false },
+        { icon: Sparkles, label: today > 0 ? "今日新帖" : "今日", value: today, hot: today > 0 },
+      ]
+    : [];
+
+  return (
+    <section className="j13-hero mb-5 lg:mb-6" aria-label="欢迎">
+      <span aria-hidden className="j13-hero-glow" />
+      <div className="relative flex flex-col lg:flex-row lg:items-center lg:justify-between gap-5">
+        <div className="min-w-0">
+          <p className="meta text-[13px]">{name ? `${hello}，${name}` : `${hello}，欢迎来到姜十三`}</p>
+          <h2
+            className="mt-1.5 text-[19px] sm:text-[22px] font-extrabold tracking-tight"
+            style={{ color: "var(--ink)" }}
+          >
+            分享技术、讨论问题、交流心得
+          </h2>
+          <div className="mt-4 flex flex-wrap gap-2.5">
+            {user ? (
+              <Link href="/compose" className="btn btn-primary w-full sm:w-auto justify-center">
+                <PenSquare size={15} /> 发起讨论
+              </Link>
+            ) : (
+              <>
+                <Link href="/login?redirect=/" className="btn btn-primary w-full sm:w-auto justify-center">
+                  <LogIn size={15} /> 登录后参与讨论
+                </Link>
+                <Link href="/register" className="btn btn-line w-full sm:w-auto justify-center">
+                  <UserPlus size={15} /> 注册
+                </Link>
+              </>
+            )}
+          </div>
+        </div>
+
+        {statItems.length > 0 && (
+          <dl className="j13-hero-stats shrink-0" aria-label="社区数据">
+            {statItems.map(({ icon: Icon, label, value, hot }) => (
+              <div key={label} className="j13-hero-stat">
+                <dd className={`j13-hero-stat-num${hot ? " is-hot" : ""}`}>{value}</dd>
+                <dt className="j13-hero-stat-lab">
+                  {hot && <span className="j13-dot-pulse" aria-hidden />}
+                  <Icon size={12} /> {label}
+                </dt>
+              </div>
+            ))}
+          </dl>
+        )}
+      </div>
+    </section>
+  );
+}