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/en/common.json

@@ -21,7 +21,7 @@
         "title": "Used by companies and people working at"
     },
     "features": {
-        "title": "Why",
+        "title": "Why choose",
         "bentoPdf": "BentoPDF?",
         "noSignup": {
             "title": "No Signup",