feat: Add Digital Signature and Validate Signature tools

New Features:
- Digital Signature tool: Sign PDFs with X.509 certificates (PFX/PEM)
  - Visible and invisible signatures
  - All pages, first page, last page, or custom page selection
  - Dynamic signature height based on text content
  - Custom signature text, image, and styling options

- Validate Signature tool: Verify digital signatures in PDFs
  - Extract and parse PKCS#7 signatures
  - View signer and issuer certificate details
  - Check certificate validity and expiry
  - Optional custom X.509 certificate for trust verification
  - Full/partial coverage detection

Infrastructure:
- Added Cloudflare Worker CORS proxy for certificate chain fetching
- Updated README with new tools and proxy deployment instructions

Documentation:
- Added Digital Signature and Validate Signature to README tools table
- Added CORS proxy deployment guide for self-hosters

src/js/config/tools.ts

@@ -726,6 +726,12 @@ export const categories = [
         icon: 'ph-certificate',
         subtitle: 'Add a cryptographic digital signature using X.509 certificates.',
       },
+      {
+        href: import.meta.env.BASE_URL + 'validate-signature-pdf.html',
+        name: 'Validate Signature',
+        icon: 'ph-seal-check',
+        subtitle: 'Verify digital signatures and view certificate details.',
+      },
     ],
   },
 ];