refactor(desktop): SaaS URL 集中配置化，消除 5 处硬编码

- 新增 .env.development / .env.production (VITE_SAAS_URL)
- saasStore.ts / LoginPage.tsx / saas-client.ts / SaaSLogin.tsx
  统一读取 import.meta.env.VITE_SAAS_URL
- 移除 LoginPage 中未使用的 isTauriRuntime import

desktop/src/lib/saas-client.ts

@@ -508,4 +508,4 @@ export interface SaaSClient {
  * Global SaaS client singleton.
  * Initialized with a default URL; the URL and token are updated on login.
  */
-export const saasClient = new SaaSClient('https://saas.zclaw.com');
+export const saasClient = new SaaSClient(import.meta.env.VITE_SAAS_URL || 'http://127.0.0.1:8080');