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/setup-checklist.tsx

@@ -114,12 +114,12 @@ export function SetupChecklist() {
             {completed}/{steps.length} completed
           </p>
         </div>
-        <Button size="sm" onClick={load} disabled={loading} className="glow">
+        <Button size="sm" onClick={load} disabled={loading}>
           {loading ? "Checking..." : "Re-check"}
         </Button>
       </div>
 
-      {error ? <div className="text-sm text-red-500">{error}</div> : null}
+      {error ? <div className="text-sm text-destructive">{error}</div> : null}
 
       <ul className="space-y-2">
         {steps.map((s) => (