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/UserList.tsx

@@ -0,0 +1,68 @@
+import Image from "next/image";
+import Link from "next/link";
+
+interface UserItem {
+  id: string;
+  username: string | null;
+  displayUsername: string | null;
+  image: string | null;
+  bio: string | null;
+}
+
+interface UserListProps {
+  users: UserItem[];
+  emptyMessage: string;
+}
+
+export function UserList({ users, emptyMessage }: UserListProps) {
+  if (users.length === 0) {
+    return (
+      <div className="text-center py-12 text-gray-500">
+        {emptyMessage}
+      </div>
+    );
+  }
+
+  return (
+    <div className="space-y-4">
+      {users.map((user) => (
+        <Link
+          key={user.id}
+          href={`/users/${user.username || user.id}`}
+          className="flex items-center gap-4 p-4 bg-white rounded-lg shadow-sm hover:shadow-md transition-shadow"
+        >
+          {user.image ? (
+            <div className="relative w-12 h-12 rounded-full overflow-hidden flex-shrink-0">
+              <Image
+                src={user.image}
+                alt={user.displayUsername || user.username || "User"}
+                fill
+                className="object-cover"
+                unoptimized
+              />
+            </div>
+          ) : (
+            <div className="w-12 h-12 rounded-full bg-primary-500 flex items-center justify-center flex-shrink-0">
+              <span className="text-lg font-bold text-white">
+                {(user.displayUsername || user.username || "U")[0].toUpperCase()}
+              </span>
+            </div>
+          )}
+          <div className="flex-1 min-w-0">
+            <div className="font-semibold text-gray-900 truncate">
+              {user.displayUsername || user.username || "Anonymous"}
+            </div>
+            {user.username && (
+              <div className="text-sm text-gray-500">@{user.username}</div>
+            )}
+            {user.bio && (
+              <div className="text-sm text-gray-600 truncate mt-1">
+                {user.bio}
+              </div>
+            )}
+          </div>
+        </Link>
+      ))}
+    </div>
+  );
+}