Add timeline view, right-click context menu, and fix Express trust proxy

- backend/server.js: Add app.set('trust proxy', 1) to fix express-rate-limit
  ValidationError when app runs behind nginx reverse proxy
- ScheduleView.jsx: Add right-click context menu on reservation blocks with
  Edit and Delete options; closes on click-outside or Escape
- ScheduleView.module.css: Add context menu styles
- TimelineView.jsx: New Gantt-style monthly timeline view showing all
  reservations sorted by date, with month navigation and right-click menu
- TimelineView.module.css: Styles for the timeline view
- App.jsx: Add 'タイムライン' tab to navigation

Co-authored-by: h <57948770+h@users.noreply.github.com>
Agent-Logs-Url: https://github.com/h/CarReservation/sessions/d03ca12c-21ce-45a0-881f-919d6635e7fb

frontend/src/App.jsx

@@ -1,6 +1,7 @@
 import { useState } from 'react';
 import ScheduleView from './components/ScheduleView.jsx';
 import CarManagement from './components/CarManagement.jsx';
+import TimelineView from './components/TimelineView.jsx';
 import styles from './App.module.css';
 
 export default function App() {
@@ -17,6 +18,12 @@ export default function App() {
           >
             📅 スケジュール
           </button>
+          <button
+            className={`${styles.navBtn} ${page === 'timeline' ? styles.active : ''}`}
+            onClick={() => setPage('timeline')}
+          >
+            📊 タイムライン
+          </button>
           <button
             className={`${styles.navBtn} ${page === 'cars' ? styles.active : ''}`}
             onClick={() => setPage('cars')}
@@ -26,7 +33,9 @@ export default function App() {
         </nav>
       </header>
       <main className={styles.main}>
-        {page === 'schedule' ? <ScheduleView /> : <CarManagement />}
+        {page === 'schedule' && <ScheduleView />}
+        {page === 'timeline' && <TimelineView />}
+        {page === 'cars' && <CarManagement />}
       </main>
     </div>
   );