feat: complete UI redesign with dark mode, sidebar navigation, and modern design system

Implemented comprehensive UI overhaul with three-layer architecture:

Layer 1 - Theme System:
- CSS variables for light/dark themes (theme.css)
- Theme composable with light/dark/auto mode (useTheme.ts)
- Sidebar state management composable (useSidebar.ts)
- Refactored main.css to use CSS variables throughout

Layer 2 - Core Components:
- AppSidebar with collapsible navigation (desktop) and overlay (mobile)
- CollapsibleSection reusable component for expandable cards
- Restructured App.vue with new sidebar layout
- Integrated Lucide icons library (lucide-vue-next)

Layer 3 - Views & Components:
- Updated all 14 views with CSS variables and responsive design
- Replaced inline SVG with Lucide icon components
- Added collapsible sections to Dashboard, Admin pages, UserProfile
- Updated 3 shared components (BookingForm, SpaceCalendar, AttachmentsList)

Features:
- Dark/light/auto theme with persistent preference
- Collapsible sidebar (icons-only on desktop, overlay on mobile)
- Consistent color palette using CSS variables
- Full responsive design across all pages
- Modern minimalist aesthetic with Indigo accent color

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/package.json

@@ -17,6 +17,7 @@
     "@fullcalendar/vue3": "^6.1.0",
     "axios": "^1.6.0",
     "chart.js": "^4.5.1",
+    "lucide-vue-next": "^0.563.0",
     "pinia": "^2.1.0",
     "vue": "^3.4.0",
     "vue-router": "^4.2.0"