feat: plans create/edit keep back and return

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

@@ -66,8 +66,12 @@
         <textarea name="description" rows="4" placeholder="可描述套餐包含的能力与适用场景">{{ old('description', $plan->description) }}</textarea>
     </label>
 
+    @php
+        $back = (string) ($back ?? '');
+        $backUrl = $back !== '' ? $back : '/admin/plans';
+    @endphp
     <div class="form-actions">
-        <a href="/admin/plans" class="btn-secondary">返回</a>
+        <a href="{{ $backUrl }}" class="btn-secondary">返回</a>
         <button type="submit">保存套餐</button>
     </div>
 </form>