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/locales/en/common.json

@@ -214,7 +214,7 @@
     "walletType": "Wallet Type",
     "walletTypeHelp": "Web3 Wallet: Polymarket accounts connected via browser wallets like MetaMask\nMagic: Polymarket accounts logged in via email or social accounts (Google, Twitter, etc.)",
     "loadingProxyOptions": "Loading proxy addresses and asset information...",
-    "selectProxyOption": "Please select a proxy address",
+    "selectProxyOption": "Please select account type",
     "proxyOptionRequired": "Please select a proxy address",
     "proxyOption": {
       "magic": {
@@ -226,6 +226,7 @@
         "title": "Safe Proxy Address"
       },
       "proxyAddress": "Proxy Address",
+      "proxyAddressHelp": "The proxy address is the wallet address you actually use on Polymarket: Magic for email/social login accounts, Safe for browser wallet (e.g. MetaMask) accounts. Please select the one that matches how you log in to Polymarket.",
       "availableBalance": "Available Balance",
       "positionBalance": "Position Balance",
       "totalBalance": "Total Balance",