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-tauri/src/settings.rs

@@ -118,7 +118,7 @@ impl AppSettings {
             .language
             .as_ref()
             .map(|s| s.trim())
-            .filter(|s| matches!(*s, "en" | "zh"))
+            .filter(|s| matches!(*s, "en" | "zh" | "ja"))
             .map(|s| s.to_string());
     }