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 />} />

admin-spa/src/components/ui/multi-select.tsx

@@ -0,0 +1,147 @@
+import * as React from "react";
+import { X, Check, ChevronsUpDown } from "lucide-react";
+import { cn } from "@/lib/utils";
+import { Badge } from "@/components/ui/badge";
+import { Button } from "@/components/ui/button";
+import {
+  Command,
+  CommandEmpty,
+  CommandGroup,
+  CommandInput,
+  CommandItem,
+} from "@/components/ui/command";
+import {
+  Popover,
+  PopoverContent,
+  PopoverTrigger,
+} from "@/components/ui/popover";
+
+export interface MultiSelectOption {
+  label: string;
+  value: string;
+}
+
+interface MultiSelectProps {
+  options: MultiSelectOption[];
+  selected: string[];
+  onChange: (selected: string[]) => void;
+  placeholder?: string;
+  emptyMessage?: string;
+  className?: string;
+  maxDisplay?: number;
+}
+
+export function MultiSelect({
+  options,
+  selected,
+  onChange,
+  placeholder = "Select items...",
+  emptyMessage = "No items found.",
+  className,
+  maxDisplay = 3,
+}: MultiSelectProps) {
+  const [open, setOpen] = React.useState(false);
+
+  const handleUnselect = (value: string) => {
+    onChange(selected.filter((s) => s !== value));
+  };
+
+  const handleSelect = (value: string) => {
+    if (selected.includes(value)) {
+      onChange(selected.filter((s) => s !== value));
+    } else {
+      onChange([...selected, value]);
+    }
+  };
+
+  const selectedOptions = options.filter((option) =>
+    selected.includes(option.value)
+  );
+
+  return (
+    <Popover open={open} onOpenChange={setOpen}>
+      <PopoverTrigger asChild>
+        <Button
+          variant="outline"
+          role="combobox"
+          aria-expanded={open}
+          className={cn(
+            "w-full justify-between h-auto min-h-10",
+            className
+          )}
+        >
+          <div className="flex gap-1 flex-wrap">
+            {selectedOptions.length === 0 && (
+              <span className="text-muted-foreground">{placeholder}</span>
+            )}
+            {selectedOptions.slice(0, maxDisplay).map((option) => (
+              <Badge
+                variant="secondary"
+                key={option.value}
+                className="mr-1 mb-1"
+                onClick={(e) => {
+                  e.stopPropagation();
+                  handleUnselect(option.value);
+                }}
+              >
+                {option.label}
+                <button
+                  className="ml-1 ring-offset-background rounded-full outline-none focus:ring-2 focus:ring-ring focus:ring-offset-2"
+                  onKeyDown={(e) => {
+                    if (e.key === "Enter") {
+                      handleUnselect(option.value);
+                    }
+                  }}
+                  onMouseDown={(e) => {
+                    e.preventDefault();
+                    e.stopPropagation();
+                  }}
+                  onClick={(e) => {
+                    e.preventDefault();
+                    e.stopPropagation();
+                    handleUnselect(option.value);
+                  }}
+                >
+                  <X className="h-3 w-3 text-muted-foreground hover:text-foreground" />
+                </button>
+              </Badge>
+            ))}
+            {selectedOptions.length > maxDisplay && (
+              <Badge
+                variant="secondary"
+                className="mr-1 mb-1"
+              >
+                +{selectedOptions.length - maxDisplay} more
+              </Badge>
+            )}
+          </div>
+          <ChevronsUpDown className="h-4 w-4 shrink-0 opacity-50" />
+        </Button>
+      </PopoverTrigger>
+      <PopoverContent className="w-full p-0" align="start">
+        <Command>
+          <CommandInput placeholder="Search..." />
+          <CommandEmpty>{emptyMessage}</CommandEmpty>
+          <CommandGroup className="max-h-64 overflow-auto">
+            {options.map((option) => (
+              <CommandItem
+                key={option.value}
+                onSelect={() => handleSelect(option.value)}
+              >
+                <Check
+                  className={cn(
+                    "mr-2 h-4 w-4",
+                    selected.includes(option.value)
+                      ? "opacity-100"
+                      : "opacity-0"
+                  )}
+                />
+                {option.label}
+              </CommandItem>
+            ))}
+          </CommandGroup>
+        </Command>
+      </PopoverContent>
+    </Popover>
+  );
+}