Implement comprehensive event series editing via modal

## Frontend Changes:
- Add EditAction enum (EditThis, EditFuture, EditAll) to event context menu
- Update context menu to show 3 edit options for recurring events
- Enhance EventCreationData with edit_scope and changed_fields tracking
- Update app component to handle EditAction types and pass to modal
- Add field change tracking infrastructure to CreateEventModal

## Backend Changes:
- Add changed_fields parameter to UpdateEventSeriesRequest for optimization
- Existing series endpoint already supports the three update types:
  - "this_only" - creates exception with EXDATE
  - "this_and_future" - creates new series with UNTIL on original
  - "all_in_series" - updates existing series in-place

## Implementation Details:
- Event context menu shows single edit option for non-recurring events
- Recurring events get three options: "Edit This Event", "Edit This and Future Events", "Edit All Events in Series"
- Modal tracks which fields user actually changed for efficient updates
- Backend series endpoint already has the logic for all three update scenarios
- Full RFC 5545 compliance with proper EXDATE and UNTIL handling

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

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

backend/src/models.rs

@@ -201,6 +201,7 @@ pub struct UpdateEventSeriesRequest {
     // Update scope control
     pub update_scope: String,      // "this_only", "this_and_future", "all_in_series"
     pub occurrence_date: Option<String>, // ISO date string for specific occurrence being updated
+    pub changed_fields: Option<Vec<String>>, // List of field names that were changed (for optimization)
 }
 
 #[derive(Debug, Serialize)]