style: streamline inputs and selects with more visible borders

- Increase --border opacity in both light and dark modes
- Harmonize Input and Select components with consistent rounded-lg corners
- Replace native selects in MainConverter with styled Select component

components/ui/Input.tsx

@@ -9,7 +9,7 @@ const Input = React.forwardRef<HTMLInputElement, InputProps>(
       <input
         type={type}
         className={cn(
-          'flex h-10 w-full rounded-xl border border-border bg-input px-4 py-2',
+          'flex h-10 w-full rounded-lg border border-border bg-input px-4 py-2',
           'text-sm ring-offset-background file:border-0 file:bg-transparent',
           'file:text-sm file:font-medium placeholder:text-muted-foreground',
           'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-primary/50 focus-visible:border-primary/50',