feat: plan store/update respect back redirect

resources/views/admin/plans/form.blade.php

@@ -12,6 +12,11 @@
 <form method="post" action="{{ $formAction }}" class="card form-grid">
     @csrf
 
+    @php $back = (string) ($back ?? ''); @endphp
+    @if($back !== '')
+        <input type="hidden" name="back" value="{{ $back }}">
+    @endif
+
     <label>
         <span>套餐名称</span>
         <input name="name" value="{{ old('name', $plan->name) }}" required>