Implement complete recurring event drag modification system

- Add recurring edit modal with three modification options:
  • "Only this event" - Creates exception for single occurrence
  • "This and future events" - Splits series from occurrence forward
  • "All occurrences in this series" - Updates entire series time

- Enhance backend update API to support series modifications:
  • Add update_action parameter for recurring event operations
  • Implement time-only updates that preserve original start dates
  • Convert timestamped occurrence UIDs to base UIDs for series updates
  • Preserve recurrence rules during series modifications

- Fix recurring event drag operations:
  • Show modal for recurring events instead of direct updates
  • Handle EXDATE creation for single occurrence modifications
  • Support series splitting with UNTIL clause modifications
  • Maintain proper UID management for different modification types

- Clean up debug logging and restore page refresh for data consistency

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

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

backend/src/models.rs

@@ -122,6 +122,8 @@ pub struct UpdateEventRequest {
     pub recurrence: String,        // recurrence type
     pub recurrence_days: Vec<bool>, // [Sun, Mon, Tue, Wed, Thu, Fri, Sat] for weekly recurrence
     pub calendar_path: Option<String>, // Optional - search all calendars if not specified
+    pub update_action: Option<String>, // "update_series" for recurring events
+    pub occurrence_date: Option<String>, // ISO date string for specific occurrence
 }
 
 #[derive(Debug, Serialize)]