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/media/FileConverter.tsx

@@ -17,7 +17,7 @@ import { addToHistory } from '@/lib/media/storage/history';
 import { downloadBlobsAsZip, generateOutputFilename } from '@/lib/media/utils/fileUtils';
 import type { ConversionJob, ConversionFormat, ConversionOptions } from '@/types/media';
 import { ShieldCheck, Download, RotateCcw, Loader2 } from 'lucide-react';
-import { cn, actionBtn } from '@/lib/utils';
+import { cn, actionBtn, cardBtn } from '@/lib/utils';
 
 type MobileTab = 'upload' | 'convert';
 
@@ -470,7 +470,7 @@ export function FileConverter() {
                   Results
                 </span>
                 {completedCount > 0 && (
-                  <button onClick={handleDownloadAll} className={actionBtn}>
+                  <button onClick={handleDownloadAll} className={cardBtn}>
                     <Download className="w-3 h-3" />
                     {completedCount > 1 ? `Download all (${completedCount}) as ZIP` : 'Download'}
                   </button>