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/Cargo.toml

@@ -22,6 +22,8 @@ web-sys = { version = "0.3", features = [
     "Document",
     "Window",
     "Location",
+    "Navigator",
+    "DomTokenList",
     "Headers",
     "Request",
     "RequestInit",