fix: Use background-blue.jpeg for AIcertifikatGablas (signatures baked in)

Same approach as ScrumMaster - signatures are already in the background image,
no need to overlay separate PNG files which had quality issues.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/services/ai-kurzy/certificate.service.js

@@ -27,7 +27,7 @@ export const CERTIFICATE_TEMPLATES = {
     name: 'AI Certifikát (Gablas)',
     file: 'AIcertifikatGablas.html',
     description: 'AI kurz - lektor Gablas + Gablasová',
-    background: 'background.jpeg',
+    background: 'background-blue.jpeg',
   },
   AIcertifikatPatrik: {
     name: 'AI Certifikát (Patrik)',