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/app.rs

@@ -1,8 +1,9 @@
 use crate::components::{
     CalendarContextMenu, ContextMenu, CreateCalendarModal, CreateEventModal, DeleteAction,
-    EditAction, EventContextMenu, EventModal, EventCreationData, ExternalCalendarModal, RouteHandler, 
-    Sidebar, Theme, ViewMode,
+    EditAction, EventContextMenu, EventModal, EventCreationData, ExternalCalendarModal, 
+    MobileWarningModal, RouteHandler, Sidebar, Theme, ViewMode,
 };
+use crate::components::mobile_warning_modal::is_mobile_device;
 use crate::components::sidebar::{Style};
 use crate::models::ical::VEvent;
 use crate::services::{calendar_service::{UserInfo, ExternalCalendar}, CalendarService};
@@ -117,6 +118,9 @@ pub fn App() -> Html {
     // External calendar state
     let external_calendars = use_state(|| -> Vec<ExternalCalendar> { Vec::new() });
     let external_calendar_events = use_state(|| -> Vec<VEvent> { Vec::new() });
+    
+    // Mobile warning state
+    let mobile_warning_open = use_state(|| is_mobile_device());
     let external_calendar_modal_open = use_state(|| false);
     let refresh_interval = use_state(|| -> Option<Interval> { None });
 
@@ -274,6 +278,13 @@ pub fn App() -> Html {
         })
     };
 
+    let on_mobile_warning_close = {
+        let mobile_warning_open = mobile_warning_open.clone();
+        Callback::from(move |_| {
+            mobile_warning_open.set(false);
+        })
+    };
+
     let on_view_change = {
         let current_view = current_view.clone();
         Callback::from(move |new_view: ViewMode| {
@@ -1676,6 +1687,12 @@ pub fn App() -> Html {
                         }
                     })}
                 />
+                
+                // Mobile warning modal
+                <MobileWarningModal
+                    is_open={*mobile_warning_open}
+                    on_close={on_mobile_warning_close}
+                />
             </div>
         </BrowserRouter>
     }