feat(frontend): 优化导入账户弹窗与文案

- 导入弹窗：增加步骤条、导入方式改为按钮、代理选项卡片紧凑展示
- 代理选项：标题改为「请选择账户类型」，地址完整显示，有资产标绿，小白说明放入卡片内
- 私钥/助记词：输入框默认两行、禁止换行（换行自动转空格/去除）
- 多语言：新增 proxyAddressHelp，更新 selectProxyOption 文案
- Modal 宽度与内边距微调；PositionList 移除未使用 useRef

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/pages/PositionList.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useState, useMemo, useRef } from 'react'
+import { useEffect, useState, useMemo } from 'react'
 import { Card, Table, Tag, message, Space, Input, Radio, Select, Button, Row, Col, Empty, Modal, Form, Descriptions } from 'antd'
 import { SearchOutlined, AppstoreOutlined, UnorderedListOutlined, UpOutlined, DownOutlined } from '@ant-design/icons'
 import { useNavigate } from 'react-router-dom'