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>

frontend/src/components/mod.rs

@@ -22,7 +22,7 @@ pub use week_view::WeekView;
 pub use event_modal::EventModal;
 pub use create_calendar_modal::CreateCalendarModal;
 pub use context_menu::ContextMenu;
-pub use event_context_menu::{EventContextMenu, DeleteAction};
+pub use event_context_menu::{EventContextMenu, DeleteAction, EditAction};
 pub use calendar_context_menu::CalendarContextMenu;
 pub use create_event_modal::{CreateEventModal, EventCreationData, EventStatus, EventClass, ReminderType, RecurrenceType};
 pub use sidebar::{Sidebar, ViewMode, Theme};