e621 polish: cached results, search context, DText descriptions

- e621 list/post queries use a 5 minute staleTime and 30 minute gcTime,
  so opening a post and coming back shows the same results with no
  refetch; global gcTime raised to 30 minutes
- post card links carry their originating search in route state; the
  detail back link returns to it and related-post links keep it
- new navigations scroll to top while history back/forward keeps the
  previous position
- DText renderer for e621 descriptions (b/i/u/s, sup/sub, code, spoiler,
  quote, color, url/wiki/thumb, headings, lists, sections, expand
  blocks, named and bare links) built as React elements, no raw HTML
- Spoiler moved to its own component; trailing punctuation no longer
  swallowed into links

frontend/src/features/online/PostCard.tsx

@@ -20,13 +20,16 @@ const ratingPill: Record<string, string> = {
 export function PostCard({
   post,
   inLibrary,
+  from,
 }: {
   post: E621Post;
   inLibrary: boolean;
+  from?: string;
 }) {
   return (
     <Link
       to={`/online/view/${post.id}`}
+      state={from ? { from } : undefined}
       className={cn(
         "group overflow-hidden rounded-lg border bg-ctp-base transition",
         ratingBorder[post.rating] ?? "border-ctp-surface0",