Add external calendars feature: display read-only ICS calendars alongside CalDAV calendars

- Database: Add external_calendars table with user relationships and CRUD operations
- Backend: Implement REST API endpoints for external calendar management and ICS fetching
- Frontend: Add external calendar modal, sidebar section with visibility toggles
- Calendar integration: Merge external events with regular events in unified view
- ICS parsing: Support multiple datetime formats, recurring events, and timezone handling
- Authentication: Integrate with existing JWT token system for user-specific calendars
- UI: Visual distinction with 📅 indicator and separate management section

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

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

frontend/src/components/mod.rs

@@ -8,6 +8,7 @@ pub mod create_event_modal;
 pub mod event_context_menu;
 pub mod event_form;
 pub mod event_modal;
+pub mod external_calendar_modal;
 pub mod login;
 pub mod month_view;
 pub mod recurring_edit_modal;
@@ -26,6 +27,7 @@ pub use create_event_modal::CreateEventModal;
 pub use event_form::EventCreationData;
 pub use event_context_menu::{DeleteAction, EditAction, EventContextMenu};
 pub use event_modal::EventModal;
+pub use external_calendar_modal::ExternalCalendarModal;
 pub use login::Login;
 pub use month_view::MonthView;
 pub use recurring_edit_modal::{RecurringEditAction, RecurringEditModal};