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/FollowButton.tsx

@@ -0,0 +1,47 @@
+"use client";
+
+import { useState, useTransition } from "react";
+import { PrimaryButton, LightButton } from "@/design-system/base/button";
+import { actionToggleFollow } from "@/modules/follow/follow-action";
+import { toast } from "sonner";
+
+interface FollowButtonProps {
+  targetUserId: string;
+  initialIsFollowing: boolean;
+  onFollowChange?: (isFollowing: boolean, followersCount: number) => void;
+}
+
+export function FollowButton({
+  targetUserId,
+  initialIsFollowing,
+  onFollowChange,
+}: FollowButtonProps) {
+  const [isFollowing, setIsFollowing] = useState(initialIsFollowing);
+  const [isPending, startTransition] = useTransition();
+
+  const handleToggleFollow = () => {
+    startTransition(async () => {
+      const result = await actionToggleFollow({ targetUserId });
+      if (result.success && result.data) {
+        setIsFollowing(result.data.isFollowing);
+        onFollowChange?.(result.data.isFollowing, result.data.followersCount);
+      } else {
+        toast.error(result.message || "Failed to update follow status");
+      }
+    });
+  };
+
+  if (isFollowing) {
+    return (
+      <LightButton onClick={handleToggleFollow} disabled={isPending}>
+        {isPending ? "..." : "Following"}
+      </LightButton>
+    );
+  }
+
+  return (
+    <PrimaryButton onClick={handleToggleFollow} disabled={isPending}>
+      {isPending ? "..." : "Follow"}
+    </PrimaryButton>
+  );
+}