feat: 实现 Builder API Key 系统级配置和 Gasless 交易支持

- 新增系统配置表（SystemConfig）用于存储 Builder API Key
- 实现 Builder API Key 的前端配置页面
- 集成 Builder Relayer API 支持 Gasless 交易
- 在 API 健康检查中添加 Builder Relayer API 检测
- 移除前端 API 测试功能，简化配置页面
- 修复赎回仓位时的 Builder API Key 检查逻辑
- 更新错误处理，引导用户配置 Builder API Key

frontend/src/App.tsx

@@ -35,6 +35,7 @@ import LanguageSettings from './pages/LanguageSettings'
 import ApiHealthStatus from './pages/ApiHealthStatus'
 import ProxySettings from './pages/ProxySettings'
 import NotificationSettings from './pages/NotificationSettings'
+import BuilderApiKeySettings from './pages/BuilderApiKeySettings'
 import { wsManager } from './services/websocket'
 import type { OrderPushMessage } from './types'
 import { apiService } from './services/api'
@@ -253,6 +254,7 @@ function App() {
           <Route path="/system-settings/api-health" element={<ProtectedRoute><ApiHealthStatus /></ProtectedRoute>} />
           <Route path="/system-settings/proxy" element={<ProtectedRoute><ProxySettings /></ProtectedRoute>} />
           <Route path="/system-settings/notifications" element={<ProtectedRoute><NotificationSettings /></ProtectedRoute>} />
+          <Route path="/system-settings/builder-api-key" element={<ProtectedRoute><BuilderApiKeySettings /></ProtectedRoute>} />
           
           {/* 默认重定向到登录页 */}
           <Route path="*" element={<Navigate to="/login" replace />} />