SightWords 1.0

Update header to full-width with centered content

- Header now spans 100% width with white background
- Title changed from "Sight Word Collections" to "Sight Words"
- Title and button are now centered horizontally
- Added border and shadow to header for definition
- Collections list remains in max-width container below

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>