feat: install @rehype-pretty/transformers + configure dual themes

- Add @rehype-pretty/transformers dependency
- Add transformerCopyButton import
- Replace github-dark single theme with github-light/github-dark-dimmed dual theme
- Add keepBackground: false, grid: true, onVisitLine fix
- Add onVisitTitle with VS Code dot indicators (red/yellow/green)
- Add transformerCopyButton with visibility hover and 2.5s feedback

package.json

@@ -10,6 +10,7 @@
   },
   "dependencies": {
     "@mdx-js/react": "^3.1.1",
+    "@rehype-pretty/transformers": "^0.13.2",
     "@wrksz/themes": "^0.9.3",
     "gray-matter": "^4.0.3",
     "katex": "^0.17.0",