feat(assets): 添加 SVG 图标和环境配置文件

- 新增 icons.svg 文件用于定义应用图标
- 添加 .env 配置文件设置 API 基础地址
- 配置图标视图框和路径数据
- 设置图标宽度为 100%，高度为 6

frontend/src/assets/icons/agent-change.svg

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