feat: add digital signature PDF tool

- Add new tool to apply cryptographic signatures to PDFs using X.509 certificates
- Support PKCS#12 (.pfx, .p12) and PEM certificate formats
- Create PKCS#7 detached signatures compatible with all major PDF viewers
- Optional visible signature with customizable position, image, and text overlay
- Add translations for English, German, Vietnamese, and Chinese

src/js/config/tools.ts

@@ -720,6 +720,12 @@ export const categories = [
         icon: 'ph-shield-check',
         subtitle: 'Set or change user permissions on a PDF.',
       },
+      {
+        href: import.meta.env.BASE_URL + 'digital-sign-pdf.html',
+        name: 'Digital Signature',
+        icon: 'ph-certificate',
+        subtitle: 'Add a cryptographic digital signature using X.509 certificates.',
+      },
     ],
   },
 ];