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

public/locales/zh/tools.json

@@ -507,5 +507,10 @@
         "contactPlaceholder": "例如：email@example.com",
         "applySignature": "应用数字签名",
         "successMessage": "PDF 签名成功！签名可在任何 PDF 阅读器中验证。"
+    },
+    "validateSignaturePdf": {
+        "name": "验证 PDF 签名",
+        "pageTitle": "验证 PDF 签名 - 验证数字签名 | BentoPDF",
+        "subtitle": "验证您的 PDF 文件中的数字签名。检查证书有效性、查看签名者详情并确认文档完整性。所有处理都在您的浏览器中进行。"
     }
 }