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

packages/browser-extension/src/ChatPanel.ts

@@ -0,0 +1,13 @@
+import { html, LitElement } from "lit";
+import { customElement } from "lit/decorators.js";
+
+@customElement("pi-chat-panel")
+export class ChatPanel extends LitElement {
+	createRenderRoot() {
+		return this;
+	}
+
+	render() {
+		return html`<h1>Hello world</h1>`;
+	}
+}