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-lock.json

@@ -15,6 +15,7 @@
         "@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"
@@ -2769,6 +2770,15 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/lucide-vue-next": {
+      "version": "0.563.0",
+      "resolved": "https://registry.npmjs.org/lucide-vue-next/-/lucide-vue-next-0.563.0.tgz",
+      "integrity": "sha512-zsE/lCKtmaa7bGfhSpN84br1K9YoQ5pCN+2oKWjQQG3Lo6ufUUKBuHSjNFI6RvUevxaajNXb8XwFUKeTXG3sIA==",
+      "license": "ISC",
+      "peerDependencies": {
+        "vue": ">=3.0.1"
+      }
+    },
     "node_modules/magic-string": {
       "version": "0.30.21",
       "resolved": "https://registry.npmjs.org/magic-string/-/magic-string-0.30.21.tgz",