feat: Simple Mode language routing and translation improvements

## Simple Mode Enhancements
- Add `simple-index.html` as dedicated homepage for Simple Mode
- Hide marketing sections (FAQ, How It Works, Related Tools) on tool pages
- Add simplified navbar and footer for tool pages in Simple Mode
- Configure vite preview server to handle language-prefixed URLs

## Language Routing
- Add middleware to rewrite language-prefixed URLs (e.g., /de/merge-pdf.html)
- Support all languages: en, de, es, fr, id, it, pt, tr, vi, zh, zh-TW
- Create .htaccess with internal rewrites for Apache/Hostinger hosting

## Translation Updates
- Add missing translations for digitalSignPdf, validateSignaturePdf,
  emailToPdf, fontToOutline, deskewPdf to es, pt, tr, zh-TW
- Add Digital Signature and Validate Signature to homepage translation keys
- Fix language regex patterns to include all supported languages
- Fix typo in encrypt-pdf.html

src/js/main.ts

@@ -273,6 +273,8 @@ const init = async () => {
     'Email to PDF': 'tools:emailToPdf',
     'Font to Outline': 'tools:fontToOutline',
     'Deskew PDF': 'tools:deskewPdf',
+    'Digital Signature': 'tools:digitalSignPdf',
+    'Validate Signature': 'tools:validateSignaturePdf',
   };
 
   // Homepage-only tool grid rendering (not used on individual tool pages)