refactor: 调整 Toast 组件在 admin 布局中的显示位置

src/routes/admin/+layout.svelte

@@ -93,7 +93,6 @@
 			<span class="user-email">{$page.data.user?.email || 'Admin'}</span>
 		</div>
 	</div>
-	<Toast />
 </header>
 
 <main id="pjax-container" style="padding-top: 58px; min-height: calc(100vh - 120px);">
@@ -102,6 +101,8 @@
 	</div>
 </main>
 
+<Toast />
+
 <style>
 	.admin-container {
 		padding-top: 2rem;