Fix divider spacing on the changelog landing page

The card grid and divider are nested inside the landing component's root
element, so Starlight's flow-spacing rule (which only targets direct
children of `.sl-markdown-content`) never applied vertical margin to the
`<hr>`. As a result the divider sat flush against the project cards.

Give the divider explicit margins using brand spacing tokens so it reads
as centered between the cards and the footer text.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: zedoraps

src/components/ChangelogLanding.astro

@@ -44,7 +44,7 @@ const hasGeneratedLanding = changelogLandingProjects.length > 0;
         ))}
       </div>
 
-      <hr />
+      <hr style="margin-block: var(--tnz-space-16) var(--tnz-space-10);" />
 
       <p>
         For general release announcements and deeper dives into selected