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/app/page.tsx

@@ -105,11 +105,11 @@ export default function Home() {
               <h4 className="font-medium mb-2">3. Try the features</h4>
               <div className="space-y-2">
                 {loading || !isAuthReady ? (
-                  <Button size="sm" className="w-full glow" disabled={true}>
+                  <Button size="sm" className="w-full" disabled={true}>
                     View Dashboard
                   </Button>
                 ) : (
-                  <Button asChild size="sm" className="w-full glow">
+                  <Button asChild size="sm" className="w-full">
                     <Link href="/dashboard">View Dashboard</Link>
                   </Button>
                 )}