feat: add cardBtn style for card title row buttons

Smaller variant for buttons that sit next to section labels in card headers
(Preview, Color, Results rows). Applied to QRPreview, FontPreview,
ColorManipulation, and FileConverter.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/qrcode/QRPreview.tsx

@@ -1,7 +1,7 @@
 'use client';
 
 import { Copy, Share2, Image as ImageIcon, FileCode, QrCode } from 'lucide-react';
-import { cn, actionBtn } from '@/lib/utils';
+import { cn, actionBtn, cardBtn } from '@/lib/utils';
 import type { ExportSize } from '@/types/qrcode';
 
 interface QRPreviewProps {
@@ -42,11 +42,11 @@ export function QRPreview({
           Preview
         </span>
 
-        <button onClick={onCopyImage} disabled={!svgString} className={actionBtn}>
+        <button onClick={onCopyImage} disabled={!svgString} className={cardBtn}>
           <Copy className="w-3 h-3" />Copy
         </button>
 
-        <button onClick={onShare} disabled={!svgString} className={actionBtn}>
+        <button onClick={onShare} disabled={!svgString} className={cardBtn}>
           <Share2 className="w-3 h-3" />Share
         </button>
 
@@ -77,7 +77,7 @@ export function QRPreview({
           </div>
         </div>
 
-        <button onClick={onDownloadSvg} disabled={!svgString} className={actionBtn}>
+        <button onClick={onDownloadSvg} disabled={!svgString} className={cardBtn}>
           <FileCode className="w-3 h-3" />SVG
         </button>
       </div>