feat: add cross-browser extension with AI reading assistant

- Create Pi Reader browser extension for Chrome/Firefox
- Chrome uses Side Panel API, Firefox uses Sidebar Action API
- Supports both browsers with separate manifests and unified codebase
- Built with mini-lit components and Tailwind CSS v4
- Features model selection dialog with Ollama support
- Hot reload development server watches both browser builds
- Add useDefineForClassFields: false to fix LitElement reactivity

.gitignore

@@ -5,6 +5,8 @@ dist/
 *.tsbuildinfo
 packages/*/node_modules/
 packages/*/dist/
+packages/*/dist-chrome/
+packages/*/dist-firefox/
 
 # Environment
 .env