Render DText to the official spec

Checked against https://e621.net/help/dtext and filled the gaps:
- backtick inline code spans and backslash-backtick escaping
- [color=...] accepts tag category names (artist, copyright, species,
  ...) alongside CSS colour names and 3/6/8-digit hex
- links: <url> brackets, "title":[url], "title":/relative paths,
  wiki links with custom titles and #anchors, {{tag search}} and entity
  references (post/topic/pool/set/comment/... #id)
- [quote=red] colours the bar instead of being read as an author
- nested lists via repeated * / # markers
- [section,expanded=Title] renders expanded
- [table] thead/tr/th/td tables and [ltable] pipe tables
- [#anchor] targets and [[#anchor]] in-page links
- headings always start their own block

frontend/src/features/online/OnlineDetailPage.tsx

@@ -308,7 +308,7 @@ export default function OnlineDetailPage() {
                   Description
                 </summary>
                 <div className="mt-2 text-sm leading-relaxed text-ctp-subtext0">
-                  {renderDText(post.description)}
+                  {renderDText(post.description, credentials?.base_url)}
                 </div>
               </details>
             </section>