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>

src/services/calendar_service.rs

@@ -193,4 +193,43 @@ impl CalendarService {
         
         grouped
     }
+
+    /// Refresh a single event by UID from the CalDAV server
+    pub async fn refresh_event(&self, token: &str, uid: &str) -> Result<Option<CalendarEvent>, String> {
+        let window = web_sys::window().ok_or("No global window exists")?;
+        
+        let opts = RequestInit::new();
+        opts.set_method("GET");
+        opts.set_mode(RequestMode::Cors);
+
+        let url = format!("{}/calendar/events/{}", self.base_url, uid);
+        let request = Request::new_with_str_and_init(&url, &opts)
+            .map_err(|e| format!("Request creation failed: {:?}", e))?;
+
+        request.headers().set("Authorization", &format!("Bearer {}", token))
+            .map_err(|e| format!("Authorization header setting failed: {:?}", e))?;
+
+        let resp_value = JsFuture::from(window.fetch_with_request(&request))
+            .await
+            .map_err(|e| format!("Network request failed: {:?}", e))?;
+
+        let resp: Response = resp_value.dyn_into()
+            .map_err(|e| format!("Response cast failed: {:?}", e))?;
+
+        let text = JsFuture::from(resp.text()
+            .map_err(|e| format!("Text extraction failed: {:?}", e))?)
+            .await
+            .map_err(|e| format!("Text promise failed: {:?}", e))?;
+
+        let text_string = text.as_string()
+            .ok_or("Response text is not a string")?;
+
+        if resp.ok() {
+            let event: Option<CalendarEvent> = serde_json::from_str(&text_string)
+                .map_err(|e| format!("JSON parsing failed: {}", e))?;
+            Ok(event)
+        } else {
+            Err(format!("Request failed with status {}: {}", resp.status(), text_string))
+        }
+    }
 }