feat: add Timestamp PDF tool with RFC 3161 support

Add document timestamping to the Secure PDF section using RFC 3161
protocol. Users can select from preset TSA servers (DigiCert, Sectigo,
SSL.com, Entrust, FreeTSA) or enter a custom TSA URL. No personal
certificate is required — only a cryptographic hash is sent to the server.

Key changes:
- Timestamp PDF page with TSA server selector, FAQ and SEO structured data
- timestampPdf() function with CORS proxy URL resolution
- TimestampNode for the workflow engine
- Tool entry in Secure PDF category + homepage i18n
- Built-in CORS proxy middleware for dev/preview
- Translations for all 16 languages

Tested with DigiCert, Sectigo and Entrust TSA servers. Timestamps are
verifiable in Adobe Acrobat (ETSI.RFC3161 SubFilter).

public/locales/en/tools.json

@@ -593,6 +593,15 @@
     "pageTitle": "Validate PDF Signature - Verify Digital Signatures | BentoPDF",
     "subtitle": "Verify digital signatures in your PDF files. Check certificate validity, view signer details, and confirm document integrity. All processing happens in your browser."
   },
+  "timestampPdf": {
+    "name": "Timestamp PDF",
+    "pageTitle": "Timestamp PDF Online Free - RFC 3161 Document Timestamp | BentoPDF",
+    "subtitle": "Add an RFC 3161 document timestamp to your PDF using a trusted Time Stamp Authority (TSA) server. Proves your document existed at a specific point in time. No certificate required.",
+    "tsaSectionTitle": "Timestamp Server (TSA)",
+    "selectTsa": "Select a TSA server",
+    "applyTimestamp": "Apply Timestamp",
+    "successMessage": "PDF timestamped successfully! The timestamp can be verified in Adobe Acrobat and other PDF readers."
+  },
   "emailToPdf": {
     "name": "Email to PDF",
     "subtitle": "Convert email files (EML, MSG) to PDF format. Supports Outlook exports and standard email formats.",