feat: replace category grid with select dropdown in units converter

- Consolidate category selection into the main converter card
- Use a space-saving Select component for category switching
- Add category color indicator to the select dropdown

components/ui/Select.tsx

@@ -18,7 +18,7 @@ const Select = React.forwardRef<HTMLSelectElement, SelectProps>(
         )}
         <select
           className={cn(
-            'flex h-10 w-full rounded-xl border border-border bg-input px-3 py-2',
+            'flex h-10 w-full rounded-lg border border-border bg-input px-3 py-2',
             'text-sm ring-offset-background',
             'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-primary/50 focus-visible:border-primary/50',
             'disabled:cursor-not-allowed disabled:opacity-50 transition-all duration-200',