style(chat): update button variants and improve file preview styling

Change button variants from 'outline' to 'default' for consistency across components. Enhance FilePreview component with better spacing, borders, and file name display to improve visual clarity and user experience.

app/components/chat/ImportFolderButton.tsx

@@ -120,13 +120,13 @@ export const ImportFolderButton: React.FC<ImportFolderButtonProps> = ({ classNam
           input?.click();
         }}
         title="Import Folder"
-        variant="outline"
+        variant="default"
         size="lg"
         className={classNames(
           'gap-2 bg-bolt-elements-background-depth-1',
           'text-bolt-elements-textPrimary',
           'hover:bg-bolt-elements-background-depth-2',
-          'border-[rgba(0,0,0,0.08)] dark:border-[rgba(255,255,255,0.08)]',
+          'border border-bolt-elements-borderColor',
           'h-10 px-4 py-2 min-w-[120px] justify-center',
           'transition-all duration-200 ease-in-out',
           className,