feat: 添加用户关注功能

- 新增 Follow 表和 User.bio 字段 (Prisma schema)
- 创建 follow 模块 (action-service-repository)
- 新增 FollowButton/FollowStats/UserList 组件
- 用户页面显示 bio、粉丝/关注数、关注按钮
- 新增 /users/[username]/followers 和 following 页面
- 添加 en-US/zh-CN i18n 翻译

⚠️ 需要运行: prisma migrate dev --name add_follow_and_bio

src/components/follow/FollowStats.tsx

@@ -0,0 +1,54 @@
+"use client";
+
+import { useState } from "react";
+import { FollowButton } from "./FollowButton";
+
+interface FollowStatsProps {
+  userId: string;
+  initialFollowersCount: number;
+  initialFollowingCount: number;
+  initialIsFollowing: boolean;
+  currentUserId?: string;
+  isOwnProfile: boolean;
+  username: string;
+}
+
+export function FollowStats({
+  userId,
+  initialFollowersCount,
+  initialFollowingCount,
+  initialIsFollowing,
+  currentUserId,
+  isOwnProfile,
+  username,
+}: FollowStatsProps) {
+  const [followersCount, setFollowersCount] = useState(initialFollowersCount);
+
+  const handleFollowChange = (isFollowing: boolean, count: number) => {
+    setFollowersCount(count);
+  };
+
+  return (
+    <div className="flex items-center gap-4">
+      <a
+        href={`/users/${username}/followers`}
+        className="text-sm text-gray-600 hover:text-primary-500 transition-colors"
+      >
+        <span className="font-semibold text-gray-900">{followersCount}</span> followers
+      </a>
+      <a
+        href={`/users/${username}/following`}
+        className="text-sm text-gray-600 hover:text-primary-500 transition-colors"
+      >
+        <span className="font-semibold text-gray-900">{initialFollowingCount}</span> following
+      </a>
+      {currentUserId && !isOwnProfile && (
+        <FollowButton
+          targetUserId={userId}
+          initialIsFollowing={initialIsFollowing}
+          onFollowChange={handleFollowChange}
+        />
+      )}
+    </div>
+  );
+}