refactor: standardize to Tailwind and shadcn default styling

- Remove all custom styling patterns and utilities (glow, gradients, custom colors)
- Fix Tailwind v4 CSS variable configuration with proper @theme directive
- Replace custom OKLCH colors with standard shadcn neutral theme
- Remove complex button styling with glow effects and gradients
- Standardize error colors to use semantic destructive tokens
- Remove custom background gradients and complex header styling
- Clean up globals.css removing custom utilities and animations
- Ensure proper dark/light mode theme switching functionality

✅ All components now use standard shadcn/Tailwind patterns
✅ Dark/light mode works correctly
✅ No custom styling or inline classes
✅ Passes lint and typecheck

🤖 Generated with Claude Code

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

src/components/site-header.tsx

@@ -9,7 +9,7 @@ export function SiteHeader() {
         <h1 className="text-2xl font-bold">
           <Link
             href="/"
-            className="bg-clip-text text-transparent [background-image:linear-gradient(90deg,color-mix(in_oklab,var(--primary)_85%,white_0%),color-mix(in_oklab,var(--primary)_50%,white_0%))] hover:opacity-90"
+            className="text-primary hover:text-primary/80 transition-colors"
           >
             Agentic Coding Boilerplate
           </Link>