fix: align Tauri arg names and improve export UX

- Match frontend camelCase keys to backend snake_case params
- Show error toast when save dialog is cancelled

src/components/SettingsModal.tsx

@@ -388,7 +388,10 @@ export default function SettingsModal({
       const defaultName = `cc-switch-config-${new Date().toISOString().split("T")[0]}.json`;
       const filePath = await window.api.saveFileDialog(defaultName);
 
-      if (!filePath) return; // 用户取消了
+      if (!filePath) {
+        onNotify?.(`${t("settings.exportFailed")}: ${t("settings.selectFileFailed")}`, "error", 4000);
+        return;
+      }
 
       const result = await window.api.exportConfigToFile(filePath);