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/FavoriteButton.tsx

@@ -0,0 +1,80 @@
+"use client";
+
+import { useState, useEffect } from "react";
+import { useRouter, usePathname } from "next/navigation";
+import { Bookmark } from "lucide-react";
+import { apiToggleFavorite } from "@/lib/api";
+import { toast } from "@/lib/toast";
+
+interface FavoriteButtonProps {
+  postId: string;
+  favorited: boolean;
+  variant?: "default" | "compact" | "article";
+}
+
+export default function FavoriteButton({
+  postId,
+  favorited: initialFavorited,
+  variant = "default",
+}: FavoriteButtonProps) {
+  const router = useRouter();
+  const pathname = usePathname();
+  const [favorited, setFavorited] = useState(initialFavorited);
+  const [loading, setLoading] = useState(false);
+
+  // router.refresh()（登录/登出后）服务端会重新下发 favorited 态，同步到本地
+  useEffect(() => {
+    setFavorited(initialFavorited);
+  }, [initialFavorited]);
+
+  const handleClick = async () => {
+    if (loading) return;
+    setLoading(true);
+    const wasFavorited = favorited;
+    setFavorited(!wasFavorited);
+    try {
+      const res = await apiToggleFavorite(postId);
+      if (res.error === "未登录") {
+        router.push(`/login?redirect=${encodeURIComponent(pathname)}`);
+        return;
+      }
+      if (typeof res.favorited === "boolean") {
+        setFavorited(res.favorited);
+      } else {
+        throw new Error(res.error || "FAIL");
+      }
+    } catch {
+      setFavorited(wasFavorited);
+      toast("操作失败，请稍后重试");
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  const sizeCls =
+    variant === "compact"
+      ? "px-2.5 py-1.5 gap-1 text-[12.5px]"
+      : variant === "article"
+        ? "px-7 py-3 gap-2 text-[15px] min-w-[132px]"
+        : "px-4 py-2 gap-1.5 text-sm";
+  const iconSize = variant === "compact" ? 13 : variant === "article" ? 18 : 15;
+
+  return (
+    <button
+      onClick={handleClick}
+      disabled={loading}
+      data-tip={favorited ? "取消收藏" : "收藏"}
+      aria-label={favorited ? "取消收藏" : "收藏"}
+      aria-pressed={favorited}
+      className={`inline-flex items-center justify-center tabular-nums rounded-full border transition-colors duration-150 disabled:opacity-50 ${
+        favorited
+          ? "border-[var(--accent)] text-[var(--accent)] bg-[color-mix(in_srgb,var(--accent)_10%,var(--panel))]"
+          : "border-[var(--line-2)] text-[var(--ink-2)] hover:border-[var(--accent)] hover:text-[var(--accent)]"
+      } ${sizeCls}`}
+      style={!favorited ? { background: "var(--panel)" } : undefined}
+    >
+      <Bookmark size={iconSize} fill={favorited ? "currentColor" : "none"} />
+      {variant === "article" && <span>{favorited ? "已收藏" : "收藏"}</span>}
+    </button>
+  );
+}