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/sidebar.rs

@@ -204,9 +204,6 @@ pub fn sidebar(props: &SidebarProps) -> Html {
                     }
                 }
             </div>
-            <nav class="sidebar-nav">
-                <Link<Route> to={Route::Calendar} classes="nav-link">{"Calendar"}</Link<Route>>
-            </nav>
             {
                 if let Some(ref info) = props.user_info {
                     if !info.calendars.is_empty() {