feat: Add coupon edit route and multiselect component

Fixed blank coupon edit page and added multiselect component

1. Fixed Missing Route:
   - Added CouponEdit import in App.tsx
   - Added route: /coupons/:id -> CouponEdit component
   - Edit page now loads correctly

2. Created MultiSelect Component:
   - Shadcn-based multiselect with search
   - Badge display for selected items
   - Click badge X to remove
   - Shows +N more when exceeds maxDisplay
   - Searchable dropdown with Command component
   - Keyboard accessible

Features:
- Selected items shown as badges
- Remove item by clicking X on badge
- Search/filter options
- Checkbox indicators
- Max display limit (default 3)
- Responsive and accessible

Next: Add product/category/brand selectors to coupon form

admin-spa/src/App.tsx

@@ -20,6 +20,7 @@ import ProductTags from '@/routes/Products/Tags';
 import ProductAttributes from '@/routes/Products/Attributes';
 import CouponsIndex from '@/routes/Coupons';
 import CouponNew from '@/routes/Coupons/New';
+import CouponEdit from '@/routes/Coupons/Edit';
 import CustomersIndex from '@/routes/Customers';
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
 import { LayoutDashboard, ReceiptText, Package, Tag, Users, Settings as SettingsIcon, Maximize2, Minimize2, Loader2 } from 'lucide-react';
@@ -477,6 +478,7 @@ function AppRoutes() {
       {/* Coupons */}
       <Route path="/coupons" element={<CouponsIndex />} />
       <Route path="/coupons/new" element={<CouponNew />} />
+      <Route path="/coupons/:id" element={<CouponEdit />} />
 
       {/* Customers */}
       <Route path="/customers" element={<CustomersIndex />} />