feat(i18n): add Vietnamese (vi) locale

Add Vietnamese translation with all 1572 keys matching en.json.
Register "vi" in SUPPORTED_LANGUAGES, LANGUAGE_FALLBACKS, and
settings dialog language type assertion. Update locale count
from 8 to 9 in docs (AGENTS.md, CONTRIBUTING.md) to account
for previously-omitted Korean (ko).

src/components/settings-dialog.tsx

@@ -483,7 +483,8 @@ export function SettingsDialog({
                 | "zh"
                 | "ja"
                 | "ko"
-                | "ru"),
+                | "ru"
+                | "vi"),
         );
         setOriginalLanguage(selectedLanguage);
       }