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/src/components/BookingForm.vue

@@ -437,7 +437,7 @@ onMounted(() => {
   display: block;
   margin-bottom: 6px;
   font-weight: 500;
-  color: #374151;
+  color: var(--color-text-primary);
   font-size: 14px;
 }
 
@@ -445,7 +445,7 @@ onMounted(() => {
   display: block;
   margin-bottom: 4px;
   font-weight: 400;
-  color: #6b7280;
+  color: var(--color-text-secondary);
   font-size: 12px;
 }
 
@@ -464,26 +464,28 @@ onMounted(() => {
 .form-textarea {
   width: 100%;
   padding: 8px 12px;
-  border: 1px solid #d1d5db;
-  border-radius: 4px;
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-sm);
   font-size: 14px;
   font-family: inherit;
-  transition: border-color 0.2s;
+  transition: border-color var(--transition-fast);
+  background: var(--color-surface);
+  color: var(--color-text-primary);
 }
 
 .form-input:focus,
 .form-textarea:focus {
   outline: none;
-  border-color: #3b82f6;
-  box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.1);
+  border-color: var(--color-accent);
+  box-shadow: 0 0 0 3px color-mix(in srgb, var(--color-accent) 15%, transparent);
 }
 
 .form-input-error {
-  border-color: #ef4444;
+  border-color: var(--color-danger);
 }
 
 .form-input:disabled {
-  background-color: #f3f4f6;
+  background-color: var(--color-bg-tertiary);
   cursor: not-allowed;
 }
 
@@ -494,24 +496,24 @@ onMounted(() => {
 .form-error {
   display: block;
   margin-top: 4px;
-  color: #ef4444;
+  color: var(--color-danger);
   font-size: 13px;
 }
 
 .api-error {
   padding: 12px;
-  background: #fee2e2;
-  color: #991b1b;
-  border-radius: 4px;
+  background: color-mix(in srgb, var(--color-danger) 10%, transparent);
+  color: var(--color-danger);
+  border-radius: var(--radius-sm);
   margin-bottom: 16px;
   font-size: 14px;
 }
 
 .success-message {
   padding: 12px;
-  background: #d1fae5;
-  color: #065f46;
-  border-radius: 4px;
+  background: color-mix(in srgb, var(--color-success) 10%, transparent);
+  color: var(--color-success);
+  border-radius: var(--radius-sm);
   margin-bottom: 16px;
   font-size: 14px;
 }
@@ -525,11 +527,11 @@ onMounted(() => {
 .btn {
   padding: 10px 20px;
   border: none;
-  border-radius: 4px;
+  border-radius: var(--radius-sm);
   font-size: 14px;
   font-weight: 500;
   cursor: pointer;
-  transition: all 0.2s;
+  transition: all var(--transition-fast);
 }
 
 .btn:disabled {
@@ -538,33 +540,34 @@ onMounted(() => {
 }
 
 .btn-primary {
-  background: #3b82f6;
+  background: var(--color-accent);
   color: white;
 }
 
 .btn-primary:hover:not(:disabled) {
-  background: #2563eb;
+  background: var(--color-accent-hover);
 }
 
 .btn-secondary {
-  background: #6b7280;
+  background: var(--color-text-secondary);
   color: white;
 }
 
 .btn-secondary:hover:not(:disabled) {
-  background: #4b5563;
+  background: var(--color-text-primary);
 }
 
 .warning-banner {
-  background: #fff3cd;
-  border: 1px solid #ffc107;
-  border-radius: 4px;
+  background: color-mix(in srgb, var(--color-warning) 15%, transparent);
+  border: 1px solid var(--color-warning);
+  border-radius: var(--radius-sm);
   padding: 12px;
   margin-bottom: 16px;
   display: flex;
   gap: 12px;
   align-items: flex-start;
   font-size: 14px;
+  color: var(--color-text-primary);
 }
 
 .warning-icon {