feat(core): add internationalization (i18n) support for console messages

[Jah-yee]

Summary

This PR adds internationalization (i18n) support to PageAgent, addressing feature request #195.

Changes

• Added i18n.ts module with translations for English and Chinese
• Updated PageAgentCore to use translator for all user-facing strings
• Support language config via AgentConfig.language (en-US/zh-CN)
• Translated console messages, observations, and error messages

Usage

import { PageAgent } from "@page-agent/core";

const agent = new PageAgent({
  language: "zh-CN", // or "en-US"
  // ...other config
});

With language: "zh-CN", console output will be in Chinese:

• 👀 观察中...
• 🧠 思考中...
• 任务完成

Related Issue

Closes #195

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