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/bak.svg

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