Add CSS guidelines to CLAUDE.md and remove todo

- Add CSS best practice to always use style.css instead of inline styles
- Remove todo for unknown bang error handling - decided to keep DuckDuckGo forwarding behavior
- DuckDuckGo fallback allows it to handle bangs not defined locally, which is more useful

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

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

Author: thavelick

CLAUDE.md

@@ -92,3 +92,8 @@ const mockWindow = {
 - `performSearch`: Hash setting and redirection
 - `setupUI`: Event listener setup and hash population
 - `initialize`: Query param detection and app initialization
+
+## Development Best Practices
+
+### CSS Guidelines
+- Always put styles in style.css, never inline