Fix: Add alarms permission and improve domain blocking

[ajczyzewska]

This PR fixes two critical issues:
Missing alarms permission

• Added "alarms" permission to manifest.json to resolve the chrome.alarms.onAlarm API error. Added fallback to setInterval if the API is unavailable.

Domain blocking not working

• Improved the blocking logic in chrome.tabs.onUpdated listener:
• Now checks both changeInfo.url and tab.url to catch all navigation events
• Added filtering for chrome://, chrome-extension://, and about: URLs
• Enhanced domain matching to handle both domain.com and www.domain.com variants
• Added proper error handling and early returns for edge cases

Changes:

• manifest.json: Added "alarms" to permissions array
• background.js:
  -- Added safeguards around chrome.alarms API usage
  -- Improved URL checking logic in tab update listener
  -- Added support for www. subdomain variants

[foull]

lgtm