refactor: web-to-pixso → web-to-ppt 全面改名

- 目录 web-to-pixso/ → web-to-ppt/
- 所有代码引用：PIXSO → WEB_TO_PPT, webToPixso → webToPPT
- 数据格式：pixso-design-capture → web-to-ppt-capture
- CSS 选择器：__web_to_pixso_ → __web_to_ppt_
- README 重写，移除 Pixso 相关描述
- docs/web-to-pixso-mapping.md → docs/web-to-ppt-mapping.md

webpack.config.js

@@ -1,10 +1,10 @@
 const path = require('path');
 
 module.exports = {
-  entry: './web-to-pixso/convert-browser.js',
+  entry: './web-to-ppt/convert-browser.js',
   output: {
     filename: 'convert-bundle.js',
-    path: path.resolve(__dirname, 'web-to-pixso'),
+    path: path.resolve(__dirname, 'web-to-ppt'),
     library: { type: 'module' },
     module: true
   },