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

tests/hooks/useSettingsForm.test.tsx

@@ -58,6 +58,29 @@ describe("useSettingsForm Hook", () => {
     expect(changeLanguageSpy).toHaveBeenCalledWith("en");
   });
 
+  it("should support japanese language preference from server data", async () => {
+    useSettingsQueryMock.mockReturnValue({
+      data: {
+        showInTray: true,
+        minimizeToTrayOnClose: true,
+        enableClaudePluginIntegration: false,
+        claudeConfigDir: "/Users/demo",
+        codexConfigDir: null,
+        language: "ja",
+      },
+      isLoading: false,
+    });
+
+    const { result } = renderHook(() => useSettingsForm());
+
+    await waitFor(() => {
+      expect(result.current.settings?.language).toBe("ja");
+    });
+
+    expect(result.current.initialLanguage).toBe("ja");
+    expect(changeLanguageSpy).toHaveBeenCalledWith("ja");
+  });
+
   it("should prioritize reading language from local storage in readPersistedLanguage", () => {
     useSettingsQueryMock.mockReturnValue({
       data: null,