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/OnlineDetailPage.tsx

@@ -8,12 +8,13 @@ import {
   HeartOff,
 } from "lucide-react";
 import { useMemo } from "react";
-import { Link, useNavigate, useParams } from "react-router-dom";
+import { Link, useLocation, useNavigate, useParams } from "react-router-dom";
 
 import { Button, EmptyState, Spinner } from "@/components/ui";
 import { TagChip } from "@/components/TagChip";
 import { api } from "@/lib/api";
 import { cn } from "@/lib/cn";
+import { renderDText } from "@/lib/dtext";
 import {
   e621ErrorMessage,
   effectiveCredentials,
@@ -46,6 +47,8 @@ export default function OnlineDetailPage() {
   const { id = "" } = useParams();
   const postId = Number(id);
   const navigate = useNavigate();
+  const location = useLocation();
+  const from = (location.state as { from?: string } | null)?.from ?? "/online";
   const queryClient = useQueryClient();
   const credentials = useE621((state) => state.credentials);
 
@@ -54,6 +57,8 @@ export default function OnlineDetailPage() {
     queryFn: ({ signal }) =>
       fetchPost(effectiveCredentials(credentials), postId, signal),
     enabled: Number.isFinite(postId) && postId > 0,
+    staleTime: 5 * 60_000,
+    gcTime: 30 * 60_000,
   });
 
   const post = query.data;
@@ -79,6 +84,8 @@ export default function OnlineDetailPage() {
     queryFn: ({ signal }) =>
       fetchPostsByIds(effectiveCredentials(credentials), relatedIds, signal),
     enabled: relatedIds.length > 0,
+    staleTime: 5 * 60_000,
+    gcTime: 30 * 60_000,
   });
 
   const lookupQuery = useQuery({
@@ -136,7 +143,7 @@ export default function OnlineDetailPage() {
     <div className="flex flex-col gap-4">
       <div className="flex items-center gap-3 text-sm">
         <Link
-          to="/online"
+          to={from}
           className="inline-flex items-center gap-1 text-ctp-subtext0 transition hover:text-ctp-text"
         >
           <ArrowLeft className="h-4 w-4" /> Online
@@ -300,9 +307,9 @@ export default function OnlineDetailPage() {
                 <summary className="cursor-pointer text-sm font-semibold text-ctp-subtext1">
                   Description
                 </summary>
-                <p className="mt-2 whitespace-pre-line text-sm text-ctp-subtext0">
-                  {post.description}
-                </p>
+                <div className="mt-2 text-sm leading-relaxed text-ctp-subtext0">
+                  {renderDText(post.description)}
+                </div>
               </details>
             </section>
           ) : null}
@@ -362,6 +369,7 @@ export default function OnlineDetailPage() {
                     <Link
                       key={related.id}
                       to={`/online/view/${related.id}`}
+                      state={{ from }}
                       className={cn(
                         "overflow-hidden rounded-md border bg-ctp-mantle transition",
                         relatedBorder[related.rating] ?? "border-ctp-surface0",

frontend/src/features/online/OnlinePage.tsx

@@ -1,6 +1,6 @@
 import { keepPreviousData, useQuery } from "@tanstack/react-query";
 import { useMemo, useState } from "react";
-import { Link, useSearchParams } from "react-router-dom";
+import { Link, useLocation, useSearchParams } from "react-router-dom";
 
 import { CollapsibleSidebar } from "@/components/CollapsibleSidebar";
 import { TagChip } from "@/components/TagChip";
@@ -23,8 +23,10 @@ const EMPTY_POSTS: E621Post[] = [];
 
 export default function OnlinePage() {
   const [searchParams, setSearchParams] = useSearchParams();
+  const location = useLocation();
   const tags = searchParams.get("tags") ?? "";
   const page = Math.max(1, Number(searchParams.get("page") ?? "1") || 1);
+  const from = `${location.pathname}${location.search}`;
 
   const credentials = useE621((state) => state.credentials);
   const blacklist = useE621((state) => state.blacklist);
@@ -41,7 +43,8 @@ export default function OnlinePage() {
         signal,
       }),
     placeholderData: keepPreviousData,
-    staleTime: 30_000,
+    staleTime: 5 * 60_000,
+    gcTime: 30 * 60_000,
   });
 
   const posts = postsQuery.data ?? EMPTY_POSTS;
@@ -270,6 +273,7 @@ export default function OnlinePage() {
                 key={post.id}
                 post={post}
                 inLibrary={inLibrary.has(post.file.md5)}
+                from={from}
               />
             ))}
           </div>

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",