feat: 前端认证网关 + API 自动附加 JWT

- AuthProvider 管理 jumpToken 交换和 JWT 生命周期
- 未授权页面（ShieldX 图标 + 提示文字）
- 加载中旋转动画
- fetchJson 全局客户端自动附加 Authorization header
- 401 响应触发重新认证
- JWT 存 sessionStorage，刷新不丢失

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/App.tsx

@@ -2,12 +2,40 @@ import { Truck, Route } from 'lucide-react';
 import { Shell, type ModuleConfig } from './components/Shell';
 import AssetsModule from './modules/assets/AssetsModule';
 import MileageModule from './modules/mileage/MileageModule';
+import AuthProvider from './auth/AuthProvider';
+import { useAuth } from './auth/useAuth';
+import UnauthorizedPage from './auth/UnauthorizedPage';
 
 const MODULES: ModuleConfig[] = [
   { id: 'assets', label: '资产管理', icon: Truck, component: AssetsModule },
   { id: 'mileage', label: '里程管理', icon: Route, component: MileageModule },
 ];
 
-export default function App() {
+function AuthGate() {
+  const { isLoading, isAuthenticated, error } = useAuth();
+
+  if (isLoading) {
+    return (
+      <div className="min-h-screen bg-[#F8F9FB] flex items-center justify-center">
+        <div className="text-center">
+          <div className="w-8 h-8 border-2 border-blue-600 border-t-transparent rounded-full animate-spin mx-auto mb-3"></div>
+          <p className="text-xs text-slate-400 font-bold">正在验证身份...</p>
+        </div>
+      </div>
+    );
+  }
+
+  if (!isAuthenticated) {
+    return <UnauthorizedPage message={error || undefined} />;
+  }
+
   return <Shell modules={MODULES} />;
 }
+
+export default function App() {
+  return (
+    <AuthProvider>
+      <AuthGate />
+    </AuthProvider>
+  );
+}