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

frontend/src/components/ScheduleView.module.css

@@ -268,3 +268,41 @@
   color: #6b7280;
   font-size: 14px;
 }
+
+/* Right-click context menu */
+.contextMenu {
+  position: fixed;
+  background: white;
+  border: 1px solid #d1d5db;
+  border-radius: 8px;
+  box-shadow: 0 4px 16px rgba(0,0,0,0.15);
+  z-index: 1000;
+  min-width: 140px;
+  overflow: hidden;
+  padding: 4px 0;
+}
+
+.contextMenuItem {
+  display: block;
+  width: 100%;
+  text-align: left;
+  background: none;
+  border: none;
+  padding: 9px 16px;
+  font-size: 13px;
+  color: #374151;
+  cursor: pointer;
+  transition: background 0.1s;
+}
+
+.contextMenuItem:hover {
+  background: #f3f4f6;
+}
+
+.contextMenuItemDelete {
+  color: #dc2626;
+}
+
+.contextMenuItemDelete:hover {
+  background: #fee2e2;
+}