Add Focus Mode feature

[smhvigorously]

Summary

• Implements a Focus Mode for distraction-blocking
• Adds dedicated CSS (css/focusMode.css) and expanded JS logic (js/focusMode.js)
• Adds UI elements to the main window and settings page

Test plan

• Toggle Focus Mode on/off from settings
• Verify blocked sites are inaccessible while focus mode is active
• Verify UI indicators appear correctly in the toolbar

🤖 Generated with Claude Code

[PalmerAL]

This seems like a neat idea, but I think we'd need to come up with a more sophisticated/accurate way of tracking whether you're on goal than the relevance scoring function in this PR. I also expect we'd want to integrate this with the existing feature named "focus mode".

I cant find it right now, but a while ago I had a branch where I tested taking the text content of the page, mapping each word to a vector in a set of word embeddings, and summing them. It was pretty accurate at identifying if two pages were related; perhaps you could do something similar here.