feat: add link prefetch for logo resource on login page (#6751)

#### What type of PR is this?

/area core
/kind improvement
/milestone 2.20.x

#### What this PR does / why we need it:

为登录页面的 Logo 资源添加 link prefetch 标签，以缓解页面抖动的问题。

before:

https://github.com/user-attachments/assets/ec718b62-6593-4deb-a028-bcb3d8504c4e

after:

https://github.com/user-attachments/assets/8d408408-37bd-4cdc-a22a-b39531fa505f

#### Does this PR introduce a user-facing change?

```release-note
None
```

application/src/main/resources/templates/gateway_modules/layout.html

@@ -10,6 +10,8 @@
         />
         <meta content="noindex,nofollow" name="robots" />
         <title th:text="${title}"></title>
+        <link rel="preload" href="/images/wordmark.svg" as="image" type="image/svg+xml" />
+        <link rel="preload" href="/images/logo.png" as="image" type="image/png" />
 
         <th:block th:replace="~{gateway_modules/common_fragments::basicStaticResources}"></th:block>