feat: 帖子收藏与用户关注

- 新增收藏功能：favorite 服务与 handler、FavoriteButton 组件
- 新增关注功能：follow 服务与 handler、FollowButton 组件
- 用户主页适配关注/粉丝与收藏展示
- flash 消息工具、广告与侧边栏组件微调

frontend/components/FollowButton.tsx

@@ -0,0 +1,77 @@
+"use client";
+
+import { useState, useEffect } from "react";
+import { useRouter, usePathname } from "next/navigation";
+import { UserPlus, UserCheck } from "lucide-react";
+import { apiToggleFollow } from "@/lib/api";
+import { toast } from "@/lib/toast";
+
+interface FollowButtonProps {
+  userId: string | number;
+  following: boolean;
+  size?: "default" | "sm";
+}
+
+export default function FollowButton({
+  userId,
+  following: initialFollowing,
+  size = "default",
+}: FollowButtonProps) {
+  const router = useRouter();
+  const pathname = usePathname();
+  const [following, setFollowing] = useState(initialFollowing);
+  const [loading, setLoading] = useState(false);
+
+  // router.refresh()（登录/登出后）服务端会重新下发关注态，同步到本地
+  useEffect(() => {
+    setFollowing(initialFollowing);
+  }, [initialFollowing]);
+
+  const handleClick = async () => {
+    if (loading) return;
+    setLoading(true);
+    const wasFollowing = following;
+    setFollowing(!wasFollowing);
+    try {
+      const res = await apiToggleFollow(userId);
+      if (res.error === "未登录") {
+        router.push(`/login?redirect=${encodeURIComponent(pathname)}`);
+        return;
+      }
+      if (typeof res.following === "boolean") {
+        setFollowing(res.following);
+        // 关注数变化后刷新页面统计（导航计数、Hero 统计）
+        router.refresh();
+      } else {
+        throw new Error(res.error || "FAIL");
+      }
+    } catch {
+      setFollowing(wasFollowing);
+      toast("操作失败，请稍后重试");
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  const sizeCls = size === "sm" ? "px-2.5 py-1 gap-1 text-[12px]" : "btn-sm";
+  const iconSize = size === "sm" ? 12 : 14;
+
+  return (
+    <button
+      onClick={handleClick}
+      disabled={loading}
+      data-tip={following ? "取消关注" : "关注"}
+      aria-label={following ? "取消关注" : "关注"}
+      aria-pressed={following}
+      className={`inline-flex items-center justify-center gap-1.5 rounded-full border font-semibold transition-colors duration-150 disabled:opacity-50 ${
+        following
+          ? "border-[var(--line-2)] text-[var(--ink-2)] hover:border-[var(--danger)] hover:text-[var(--danger)]"
+          : "border-[var(--accent)] text-[var(--accent)] hover:bg-[color-mix(in_srgb,var(--accent)_10%,var(--panel))]"
+      } ${sizeCls}`}
+      style={{ background: "var(--panel)" }}
+    >
+      {following ? <UserCheck size={iconSize} /> : <UserPlus size={iconSize} />}
+      {following ? "已关注" : "关注"}
+    </button>
+  );
+}