feat(i18n): Add Vietnamese (Tiếng Việt) language support

- Add Vietnamese translation files (common.json and tools.json)
- Register 'vi' as a supported language in i18n configuration
- Update language switcher to support Vietnamese footer text
- Update URL routing regex to handle /vi/ paths

All 170 translation keys have been translated to Vietnamese.
Users can now access BentoPDF in Vietnamese by visiting /vi/ URLs.

public/locales/de/common.json

@@ -22,7 +22,7 @@
     },
     "features": {
         "title": "Warum",
-        "bentoPdf": "BentoPDF?",
+        "bentoPdf": "BentoPDF wählen?",
         "noSignup": {
             "title": "Keine Anmeldung",
             "description": "Sofort starten, keine Konten oder E-Mails erforderlich."