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/background.ts

@@ -0,0 +1,23 @@
+// Declare browser global for Firefox
+declare const browser: any;
+
+// Detect browser type
+const isFirefox = typeof browser !== "undefined" && typeof browser.runtime !== "undefined";
+const browserAPI = isFirefox ? browser : chrome;
+
+// Open side panel/sidebar when extension icon is clicked
+browserAPI.action.onClicked.addListener((tab: chrome.tabs.Tab) => {
+	if (isFirefox) {
+		// Firefox: Toggle the sidebar
+		if (typeof browser !== "undefined" && browser.sidebarAction) {
+			browser.sidebarAction.toggle();
+		}
+	} else {
+		// Chrome: Open the side panel
+		if (tab.id && chrome.sidePanel) {
+			chrome.sidePanel.open({ tabId: tab.id });
+		}
+	}
+});
+
+export {};