Code blocks and timestamps now render correctly across timezones

Two reader-facing rendering bugs got fixed in this release.

Code blocks were sometimes running together with the paragraph that followed them because of a missing newline in the markdown rendering pipeline. The result was visually confusing — your prose blended into a code block. Both the markdown renderer and the editor's serializer now emit the right whitespace.

Separately, article timestamps were rendering differently between server and client because the client was using the visitor's local timezone while the server defaulted to UTC. The mismatch caused a brief visual flicker on first load. We've pinned timestamp rendering to UTC on both ends, so the date stays put.

While we were in the renderer we also added proper styling for h1 elements with scroll-margin so anchor links to headings land in the right spot.