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/App.tsx

@@ -478,7 +478,7 @@ function AppRoutes() {
       {/* Coupons */}
       <Route path="/coupons" element={<CouponsIndex />} />
       <Route path="/coupons/new" element={<CouponNew />} />
-      <Route path="/coupons/:id" element={<CouponEdit />} />
+      <Route path="/coupons/:id/edit" element={<CouponEdit />} />
 
       {/* Customers */}
       <Route path="/customers" element={<CustomersIndex />} />