style(theme): 优化SVG图标并新增暗色主题配置

- 优化了download、menu、minus-sign、plus、reply等SVG图标文件，移除了冗余的XML声明和命名空间
- 新增了6套暗色AI主题配置，包括琥珀流光、翡翠流光、翡翠森林、深海幽蓝、玫瑰绯红、紫罗兰梦
- 更新了README.md文档，改进了标题层级结构和命令说明章节
- 添加了详细的开发、构建、辅助命令说明和使用指南

cutejava-front/src/assets/icons/svg/add.svg

@@ -1,2 +1 @@
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+<svg class="icon" viewBox="0 0 1024 1024" xmlns="http://www.w3.org/2000/svg" width="200" height="200"><defs><style>@font-face{font-family:rbicon;src:url(chrome-extension://dipiagiiohfljcicegpgffpbnjmgjcnf/fonts/rbicon.woff2) format(&quot;woff2&quot;);font-weight:400;font-style:normal}</style></defs><path d="M590.77 571.077h324.922c15.754 0 29.539-13.785 29.539-29.539v-59.076c0-15.754-13.785-29.539-29.539-29.539H590.77c-11.815 0-19.692-7.877-19.692-19.692V108.308c0-15.754-13.785-29.539-29.539-29.539h-59.076c-15.754 0-29.539 13.785-29.539 29.539V433.23c0 11.815-7.877 19.692-19.692 19.692H108.308c-15.754 0-29.539 13.785-29.539 29.539v59.076c0 15.754 13.785 29.539 29.539 29.539H433.23c11.815 0 19.692 7.877 19.692 19.692v324.923c0 15.754 13.785 29.539 29.539 29.539h59.076c15.754 0 29.539-13.785 29.539-29.539V590.77c0-11.815 7.877-19.692 19.692-19.692z"/></svg>