feat: enhance header component with complete navigation structure

- Add responsive header with mobile hamburger menu
- Implement user authentication state handling
- Style with primary color scheme using neutral/primary variants
- Add proper navigation links for events and concerts sections
- Include user dropdown menu with profile and logout options

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

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

app/assets/stylesheets/theme.css

@@ -100,11 +100,6 @@
 }
 
 /* Base styles */
-* {
-  margin: 0;
-  padding: 0;
-  box-sizing: border-box;
-}
 
 body {
   font-family: var(--font-sans);