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.

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "bento-pdf",
-  "version": "1.10.5",
+  "version": "1.11.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "bento-pdf",
-      "version": "1.10.5",
+      "version": "1.11.1",
       "license": "Apache-2.0",
       "dependencies": {
         "@fontsource/cedarville-cursive": "^5.2.7",