Fix frontend to use series endpoint for single occurrence modifications

- Extended update callback signature to include update_scope and occurrence_date parameters
- Modified app.rs to detect when series endpoint should be used vs regular endpoint
- Updated calendar_service to automatically set occurrence_date for "this_only" updates
- Modified all callback emit calls throughout frontend to include new parameters
- Week_view now properly calls series endpoint with "this_only" scope for single occurrence edits

This ensures that single occurrence modifications (RecurringEditAction::ThisEvent)
now go through the proper series endpoint which will:
- Add EXDATE to the original recurring series
- Create exception event with RECURRENCE-ID
- Show proper debug logging in backend

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

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

frontend/src/services/calendar_service.rs

@@ -929,8 +929,13 @@ impl CalendarService {
                 "recurrence_end_date": until_date.as_ref().map(|dt| dt.format("%Y-%m-%d").to_string()),
                 "recurrence_count": None as Option<u32>, // No count limit by default
                 "calendar_path": calendar_path,
-                "update_scope": update_scope,
-                "occurrence_date": None as Option<String> // For specific occurrence updates
+                "update_scope": update_scope.clone(),
+                "occurrence_date": if update_scope == "this_only" { 
+                    // For single occurrence updates, use the original event's start date as occurrence_date
+                    Some(start_date.clone())
+                } else { 
+                    None 
+                }
             });
             let url = format!("{}/calendar/events/series/update", self.base_url);
             (body, url)