feat(i18n): add Japanese language support

- Add complete Japanese translation file (ja.json)
- Update frontend types and hooks to support "ja" language option
- Add Japanese tray menu texts in Rust backend
- Add Japanese option to language settings component
- Update Zod schema to include Japanese language validation
- Add test case for Japanese language preference
- Update i18n documentation to reflect three-language support

src/hooks/useSettings.ts

@@ -12,7 +12,7 @@ import {
 } from "./useDirectorySettings";
 import { useSettingsMetadata } from "./useSettingsMetadata";
 
-type Language = "zh" | "en";
+type Language = "zh" | "en" | "ja";
 
 interface SaveResult {
   requiresRestart: boolean;