Major CSS cleanup and mobile detection system

CSS Improvements:
- Remove all mobile responsive CSS (@media queries) - 348+ lines removed
- Add comprehensive CSS variables for glass effects, control dimensions, transitions
- Consolidate duplicate patterns (43+ transition, 37+ border-radius, 61+ padding instances)
- Remove legacy week grid CSS section
- Reduce total CSS from 4,197 to 3,828 lines (8.8% reduction)

Sidebar Enhancements:
- Remove unused sidebar-nav div and navigation link
- Standardize all dropdown controls to consistent 40px height and styling
- Reduce calendar item padding from 0.75rem to 0.5rem for more compact display
- Unify theme-selector and style-selector styling with view-selector

Mobile Detection:
- Add MobileWarningModal component with device detection
- Show helpful popup directing mobile users to native CalDAV apps
- Add Navigator and DomTokenList web-sys features
- Desktop-focused experience with appropriate mobile guidance

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

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

frontend/src/components/mod.rs

@@ -10,6 +10,7 @@ pub mod event_form;
 pub mod event_modal;
 pub mod external_calendar_modal;
 pub mod login;
+pub mod mobile_warning_modal;
 pub mod month_view;
 pub mod recurring_edit_modal;
 pub mod route_handler;
@@ -29,6 +30,7 @@ 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;
 pub use recurring_edit_modal::{RecurringEditAction, RecurringEditModal};
 pub use route_handler::RouteHandler;