add display css variable for html preview link (#1412)

Adds the `display` property to the `a.htmlrunner-link` class for
`HtmlRunner`.
This is to enable / disable the HTML preview link. 
The link is currently broken in CCP due to the use of relative link for
the preview and we would like to disable it until it gets fixed
(hopefully in the issue
RaspberryPiFoundation/digital-editor-issues#1264)

Before:
<img width="600" alt="Screenshot 2026-03-31 at 11 39 23"
src="https://github.com/user-attachments/assets/f002c4bb-9c93-45f5-a32d-f98b89bc75e4"
/>
<img width="500" alt="Screenshot 2026-03-31 at 11 51 24"
src="https://github.com/user-attachments/assets/39800ecb-1424-48d0-81d9-abac85d4e34f"
/>



After with `--editor-htmlrunner-link-display: none`:
<img width="600" alt="Screenshot 2026-03-31 at 11 39 46"
src="https://github.com/user-attachments/assets/78a3f2cc-642e-45e3-b838-dd455e9d62e3"
/>
<img width="500" alt="Screenshot 2026-03-31 at 11 51 55"
src="https://github.com/user-attachments/assets/a3e5bdb7-07ee-4289-9579-b3f639fea664"
/>

Author: cocomarine

src/assets/stylesheets/HtmlRunner.scss

@@ -8,7 +8,8 @@
   inline-size: 100%;
 }
 
-.htmlrunner-link {
+a.htmlrunner-link {
+  display: var(--editor-htmlrunner-link-display, inline-flex);
   margin-inline-start: auto;
 }