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: pdf114514 <57948770+pdf114514@users.noreply.github.com>
Agent-Logs-Url: https://github.com/pdf114514/CarReservation/sessions/d03ca12c-21ce-45a0-881f-919d6635e7fb

backend/server.js

@@ -7,6 +7,10 @@ const path = require('path');
 const app = express();
 const PORT = process.env.PORT || 3001;
 
+// Trust the first proxy (nginx) so that express-rate-limit can correctly
+// identify clients by their real IP from the X-Forwarded-For header.
+app.set('trust proxy', 1);
+
 app.use(cors());
 app.use(express.json());