refactor: Standardize edit routes to /{entity}/{id}/edit

Consistency fix: All edit routes now follow same pattern

Before:
- Products: /products/123/edit ✅
- Orders: /orders/123/edit ✅
- Coupons: /coupons/123 ❌ (inconsistent)

After:
- Products: /products/123/edit ✅
- Orders: /orders/123/edit ✅
- Coupons: /coupons/123/edit ✅ (now consistent)

Changes:
1. App.tsx - Route: /coupons/:id → /coupons/:id/edit
2. Coupons/index.tsx - Link: /coupons/${id} → /coupons/${id}/edit

Benefits:
✅ Consistent URL pattern across all entities
✅ Clear intent (edit vs detail)
✅ Easier to add detail pages later if needed
✅ Follows REST conventions

Note: Even though coupons/products have no detail page in admin,
using /edit suffix maintains consistency and allows future expansion.

admin-spa/src/routes/Coupons/index.tsx

@@ -253,7 +253,7 @@ export default function CouponsIndex() {
                     />
                   </td>
                   <td className="p-3">
-                    <Link to={`/coupons/${coupon.id}`} className="font-medium hover:underline">
+                    <Link to={`/coupons/${coupon.id}/edit`} className="font-medium hover:underline">
                         {coupon.code}
                     </Link>
                     {coupon.description && (