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/components/LoginPage.tsx

@@ -22,12 +22,10 @@ import {
 } from 'lucide-react';
 import { useSaaSStore } from '../store/saasStore';
 import { cn } from '../lib/utils';
-import { isTauriRuntime } from '../lib/tauri-gateway';
 
 // === Constants ===
 
-const PRODUCTION_SAAS_URL = 'https://saas.zclaw.com';
-const DEV_SAAS_URL = 'http://127.0.0.1:8080';
+const SAAS_URL = import.meta.env.VITE_SAAS_URL || 'http://127.0.0.1:8080';
 
 // === Animation Variants ===
 
@@ -86,8 +84,7 @@ function ZclawLogo({ className }: { className?: string }) {
 
 /** 根据运行环境自动选择 SaaS 服务器地址 */
 function getSaasUrl(): string {
-  if (import.meta.env.DEV) return DEV_SAAS_URL;
-  return isTauriRuntime() ? PRODUCTION_SAAS_URL : DEV_SAAS_URL;
+  return SAAS_URL;
 }
 
 // === Component ===

desktop/src/components/SaaS/SaaSLogin.tsx

@@ -246,7 +246,7 @@ export function SaaSLogin({ onLogin, onLoginWithTotp, onRegister, initialUrl, is
                   type="url"
                   value={serverUrl}
                   onChange={(e) => setServerUrl(e.target.value)}
-                  placeholder="https://saas.zclaw.com"
+                  placeholder={import.meta.env.VITE_SAAS_URL || 'http://127.0.0.1:8080'}
                   className="w-full pl-10 pr-3 py-2 border border-gray-300 rounded-lg text-sm focus:outline-none focus:ring-2 focus:ring-emerald-500/20 focus:border-emerald-500 bg-white text-gray-900"
                   disabled={isLoggingIn}
                 />