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/components/settings/LanguageSettings.tsx

@@ -2,9 +2,11 @@ import { Button } from "@/components/ui/button";
 import { cn } from "@/lib/utils";
 import { useTranslation } from "react-i18next";
 
+type LanguageOption = "zh" | "en" | "ja";
+
 interface LanguageSettingsProps {
-  value: "zh" | "en";
-  onChange: (value: "zh" | "en") => void;
+  value: LanguageOption;
+  onChange: (value: LanguageOption) => void;
 }
 
 export function LanguageSettings({ value, onChange }: LanguageSettingsProps) {
@@ -25,6 +27,9 @@ export function LanguageSettings({ value, onChange }: LanguageSettingsProps) {
         <LanguageButton active={value === "en"} onClick={() => onChange("en")}>
           {t("settings.languageOptionEnglish")}
         </LanguageButton>
+        <LanguageButton active={value === "ja"} onClick={() => onChange("ja")}>
+          {t("settings.languageOptionJapanese")}
+        </LanguageButton>
       </div>
     </section>
   );