Implement real-time event refresh functionality

- Backend: Add GET /api/calendar/events/:uid endpoint for single event refresh
- Backend: Implement fetch_event_by_uid method to retrieve updated events from CalDAV
- Frontend: Add event click callback system to trigger refresh on interaction
- Frontend: Display loading state with orange pulsing animation during refresh
- Frontend: Smart event data updates without full calendar reload
- Frontend: Graceful error handling with fallback to cached data
- CSS: Add refreshing animation for visual feedback during updates

Events now automatically refresh from CalDAV server when clicked, ensuring
users always see the most current event data including changes made in
other calendar applications.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

styles.css

@@ -359,6 +359,20 @@ body {
     background: #1976d2;
 }
 
+.event-box.refreshing {
+    background: #ff9800;
+    animation: pulse 1.5s ease-in-out infinite alternate;
+}
+
+@keyframes pulse {
+    0% {
+        opacity: 0.7;
+    }
+    100% {
+        opacity: 1;
+    }
+}
+
 .event-dot {
     background: #ff9800;
     height: 6px;