Enhance: plans index show safe back link

tests/Feature/AdminPlanIndexBackLinkTest.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Tests\Feature;
+
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Tests\TestCase;
+
+class AdminPlanIndexBackLinkTest extends TestCase
+{
+    use RefreshDatabase;
+
+    protected function loginAsPlatformAdmin(): void
+    {
+        $this->seed();
+
+        $this->post('/admin/login', [
+            'email' => 'platform.admin@demo.local',
+            'password' => 'Platform@123456',
+        ])->assertRedirect('/admin');
+    }
+
+    public function test_index_should_show_safe_back_link_when_back_is_relative_path(): void
+    {
+        $this->loginAsPlatformAdmin();
+
+        $this->get('/admin/plans?status=active&back=' . urlencode('/admin/site-subscriptions?status=activated'))
+            ->assertOk()
+            ->assertSee('返回上一页（保留上下文）')
+            ->assertSee('href="/admin/site-subscriptions?status=activated"', false);
+    }
+
+    public function test_index_should_not_show_back_link_when_back_is_external_url(): void
+    {
+        $this->loginAsPlatformAdmin();
+
+        $this->get('/admin/plans?back=' . urlencode('https://evil.example.com/'))
+            ->assertOk()
+            ->assertDontSee('返回上一页（保留上下文）');
+    }
+}