style(components): refine core UI components for minimal aesthetics

- card: switched to subtle borders (border-white/5)
- input: increased height to h-10, enhanced focus transitions, removed internal shadows
- button: removed default shadows for flatter appearance

src/components/ui/card.tsx

@@ -5,7 +5,7 @@ const Card = React.forwardRef<HTMLDivElement, React.HTMLAttributes<HTMLDivElemen
   ({ className, ...props }, ref) => (
     <div
       ref={ref}
-      className={cn('bg-card text-card-foreground rounded-xl border shadow', className)}
+      className={cn('bg-card text-card-foreground rounded-xl border border-white/5 shadow-sm', className)}
       {...props}
     />
   ),