Implement car reservation schedule management system

Co-authored-by: h <57948770+h@users.noreply.github.com>
Agent-Logs-Url: https://github.com/h/CarReservation/sessions/1d8c6b05-0e8d-4484-a2d8-8d427dfad9cb

frontend/src/App.jsx

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+import { useState } from 'react';
+import ScheduleView from './components/ScheduleView.jsx';
+import CarManagement from './components/CarManagement.jsx';
+import styles from './App.module.css';
+
+export default function App() {
+  const [page, setPage] = useState('schedule');
+
+  return (
+    <div className={styles.app}>
+      <header className={styles.header}>
+        <h1 className={styles.title}>🚗 代車スケジュール管理</h1>
+        <nav className={styles.nav}>
+          <button
+            className={`${styles.navBtn} ${page === 'schedule' ? styles.active : ''}`}
+            onClick={() => setPage('schedule')}
+          >
+            📅 スケジュール
+          </button>
+          <button
+            className={`${styles.navBtn} ${page === 'cars' ? styles.active : ''}`}
+            onClick={() => setPage('cars')}
+          >
+            🚙 代車管理
+          </button>
+        </nav>
+      </header>
+      <main className={styles.main}>
+        {page === 'schedule' ? <ScheduleView /> : <CarManagement />}
+      </main>
+    </div>
+  );
+}