Fix OAuth account connection hanging with "Background script not responding"

[Copilot]

After completing the Google OAuth flow, the extension would hang for ~30 seconds before showing "Background script not responding" — instead of connecting the account. The background service worker was silently returning in failure cases without notifying the options page, leaving it waiting indefinitely.

Changes

• Missing early return on isWorking guard — error was sent but execution fell through and launched a second concurrent auth flow; added return to exit cleanly.

• Silent returns replaced with explicit errors — two cases where the background returned without messaging the options page:

  • launchWebAuthFlow resolving to undefined (cancelled, blocked, or popup closed mid-flow)
  • Token extraction failure

• Fragile regex → URLSearchParams — old regex /\#(?:access_token)\=([\S\s]*?)\&/ required a trailing &, silently breaking if access_token is the last fragment parameter (parameter ordering is not guaranteed by Google):

// Before — breaks if access_token is last in fragment
const tokenMatch = redirectTokenUrl.match(/\#(?:access_token)\=([\S\s]*?)\&/);
const token = tokenMatch[1];

// After — order-independent, standards-compliant
const redirectUrl = new URL(redirectTokenUrl);
const hashParams = new URLSearchParams(redirectUrl.hash.substring(1));
const token = hashParams.get("access_token");