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/close.svg

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+<?xml version="1.0" standalone="no"?><!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd"><svg t="1764755052115" class="icon" viewBox="0 0 1024 1024" version="1.1" xmlns="http://www.w3.org/2000/svg" p-id="6234" xmlns:xlink="http://www.w3.org/1999/xlink" width="16" height="16"><path d="M886.784 746.496q29.696 30.72 43.52 56.32t-4.608 58.368q-4.096 6.144-11.264 14.848t-14.848 16.896-15.36 14.848-12.8 9.728q-25.6 15.36-60.416 8.192t-62.464-34.816l-43.008-43.008-57.344-57.344-67.584-67.584-73.728-73.728-131.072 131.072q-60.416 60.416-98.304 99.328-38.912 38.912-77.312 48.128t-68.096-17.408l-7.168-7.168-11.264-11.264-11.264-11.264q-6.144-6.144-7.168-8.192-11.264-14.336-13.312-29.184t2.56-29.184 13.824-27.648 20.48-24.576q9.216-8.192 32.768-30.72l55.296-57.344q33.792-32.768 75.264-73.728t86.528-86.016q-49.152-49.152-93.696-93.184t-79.872-78.848-57.856-56.832-27.648-27.136q-26.624-26.624-27.136-52.736t17.92-52.736q8.192-10.24 23.552-24.064t21.504-17.92q30.72-20.48 55.296-17.92t49.152 28.16l31.744 31.744q23.552 23.552 58.368 57.344t78.336 76.288 90.624 88.576q38.912-38.912 76.288-75.776t69.632-69.12 58.368-57.856 43.52-43.008q24.576-23.552 53.248-31.232t55.296 12.8q1.024 1.024 6.656 5.12t11.264 9.216 10.752 9.728 7.168 5.632q27.648 26.624 27.136 57.856t-27.136 57.856q-18.432 18.432-45.568 46.08t-60.416 60.416-70.144 69.632l-77.824 77.824q37.888 36.864 74.24 72.192t67.584 66.048 56.32 56.32 41.472 41.984z" p-id="6235"></path></svg>