Implement comprehensive theme system with FontAwesome icons

- Add comprehensive CSS custom properties for all theme colors
- Include modal, button, input, text, and background color variables
- Enhance dark theme with complete variable overrides for proper contrast
- Replace hardcoded colors in print-preview.css with theme variables
- Add FontAwesome CDN integration and replace emoji icons
- Create minimalistic glass-effect checkbox styling with transparency
- Fix white-on-white text issue in dark theme across all modals

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

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

frontend/src/components/calendar_header.rs

@@ -45,7 +45,7 @@ pub fn calendar_header(props: &CalendarHeaderProps) -> Html {
                     if let Some(print_callback) = &props.on_print {
                         html! {
                             <button class="print-button" onclick={print_callback.clone()} title="Print Calendar">
-                                {"🖨️"}
+                                <i class="fas fa-print"></i>
                             </button>
                         }
                     } else {

frontend/src/components/sidebar.rs

@@ -350,9 +350,9 @@ pub fn sidebar(props: &SidebarProps) -> Html {
                                                         >
                                                             {
                                                                 if props.refreshing_calendar_id == Some(cal.id) {
-                                                                    "⏳" // Loading spinner
+                                                                    html! { <i class="fas fa-spinner fa-spin"></i> }
                                                                 } else {
-                                                                    "🔄" // Normal refresh icon
+                                                                    html! { <i class="fas fa-sync-alt"></i> }
                                                                 }
                                                             }
                                                         </button>