Fix: guard screen access for service worker context

The background.js service worker does not have access to the window
screen object. Accessing screen.width threw a ReferenceError which
silently prevented startSession() from completing and blocked all
annotation saves.

Added typeof screen !== 'undefined' guard so screenResolution falls
back to 'N/A' when running in service worker context.

https://claude.ai/code/session_013y1QH2gkj8wg427JpzjdJe

Author: claude

src/browserInfo.js

@@ -44,7 +44,7 @@ export async function getSystemInfo() {
         browserVersion,
         os,
         osVersion,
-        screenResolution: `${screen.width} × ${screen.height}`,
+        screenResolution: (typeof screen !== 'undefined' && screen.width) ? `${screen.width} × ${screen.height}` : 'N/A',
         language: navigator.language,
         timezone: Intl.DateTimeFormat().resolvedOptions().timeZone,
         cookies: navigator.cookieEnabled