Add header transparency and glassmorphism effect

- Set header background to 90% opacity for subtle transparency
- Add backdrop-blur-sm for glassmorphism effect
- Update header border to use CSS variable for proper dark mode support
- Content now shows through header when scrolling for better visual continuity

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

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

Author: groksrc

src/components/Header.astro

@@ -5,7 +5,7 @@ import Search from './Search.astro'
 import { navConfig } from '@/config/navigation'
 ---
 
-<header class="sticky top-0 z-50 w-full border-b border-gray-100 bg-background">
+<header class="sticky top-0 z-50 w-full border-b border-border bg-background/90 backdrop-blur-sm">
   <div class="flex justify-center">
     <div class="flex h-14 w-full max-w-screen-2xl items-center px-4 md:px-0">
       <!-- Logo -->