Implement tabbed calendar management modal with improved styling

- Replace separate Create Calendar and External Calendar modals with unified tabbed interface
- Redesign modal styling with less rounded corners and cleaner appearance
- Significantly increase padding throughout modal components for better spacing
- Fix CSS variable self-references (control-padding, standard-transition)
- Improve button styling with better padding (0.875rem 2rem) and colors
- Enhance form elements with generous padding (1rem) and improved focus states
- Redesign tab bar with segmented control appearance and proper active states
- Update context menus with modern glassmorphism styling and smooth animations
- Consolidate calendar management functionality into single reusable component

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

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

frontend/src/components/mod.rs

@@ -1,5 +1,6 @@
 pub mod calendar;
 pub mod calendar_context_menu;
+pub mod calendar_management_modal;
 pub mod calendar_header;
 pub mod calendar_list_item;
 pub mod context_menu;
@@ -19,16 +20,15 @@ pub mod week_view;
 
 pub use calendar::Calendar;
 pub use calendar_context_menu::CalendarContextMenu;
+pub use calendar_management_modal::CalendarManagementModal;
 pub use calendar_header::CalendarHeader;
 pub use calendar_list_item::CalendarListItem;
 pub use context_menu::ContextMenu;
-pub use create_calendar_modal::CreateCalendarModal;
 pub use create_event_modal::CreateEventModal;
 // Re-export event form types for backwards compatibility
 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 mobile_warning_modal::MobileWarningModal;
 pub use month_view::MonthView;