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

prisma/schema.prisma

@@ -17,12 +17,15 @@ model User {
   updatedAt            DateTime             @updatedAt
   displayUsername      String?
   username             String               @unique
+  bio                  String?
   accounts             Account[]
   dictionaryLookUps    DictionaryLookUp[]
   folders              Folder[]
   folderFavorites      FolderFavorite[]
   sessions             Session[]
   translationHistories TranslationHistory[]
+  followers            Follow[]             @relation("UserFollowers")
+  following            Follow[]             @relation("UserFollowing")
 
   @@map("user")
 }
@@ -198,3 +201,18 @@ model TranslationHistory {
   @@index([translatedText, sourceLanguage, targetLanguage])
   @@map("translation_history")
 }
+
+model Follow {
+  id          String   @id @default(cuid())
+  followerId  String   @map("follower_id")
+  followingId String   @map("following_id")
+  createdAt   DateTime @default(now()) @map("created_at")
+
+  follower  User @relation("UserFollowers", fields: [followerId], references: [id], onDelete: Cascade)
+  following User @relation("UserFollowing", fields: [followingId], references: [id], onDelete: Cascade)
+
+  @@unique([followerId, followingId])
+  @@index([followerId])
+  @@index([followingId])
+  @@map("follows")
+}