feat: add hidden /portal page for SinoCode iframe embedding

Full-screen landing page with glassmorphism cards linking to monitoring
(8018), docs (8017), and analytics (8019). Sidebar is conditionally
hidden on the portal route. Supports dark/light themes and i18n.

lib/i18n.tsx

@@ -80,6 +80,15 @@ const translations = {
     "theme.light": "浅色",
     "theme.dark": "深色",
     "theme.system": "系统",
+    // portal
+    "portal.subtitle": "智能 API 网关平台",
+    "portal.monitoring": "监控站",
+    "portal.monitoringDesc": "实时系统状态监控与告警通知",
+    "portal.docs": "文档中心",
+    "portal.docsDesc": "API 接口文档与集成指南",
+    "portal.analytics": "使用统计",
+    "portal.analyticsDesc": "API 调用数据分析与可视化看板",
+    "portal.enter": "进入",
   },
   en: {
     "nav.overview": "Overview",
@@ -144,6 +153,14 @@ const translations = {
     "theme.light": "Light",
     "theme.dark": "Dark",
     "theme.system": "System",
+    "portal.subtitle": "Intelligent API Gateway Platform",
+    "portal.monitoring": "Monitoring",
+    "portal.monitoringDesc": "Real-time system status monitoring & alerts",
+    "portal.docs": "Documentation",
+    "portal.docsDesc": "API reference docs & integration guides",
+    "portal.analytics": "Usage Analytics",
+    "portal.analyticsDesc": "API usage data analytics & visualization dashboard",
+    "portal.enter": "Enter",
   },
 } as const;