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/components/AppShell.tsx

@@ -1,7 +1,7 @@
 import { useQuery } from "@tanstack/react-query";
 import { LogOut, Menu } from "lucide-react";
 import { useEffect, useState } from "react";
-import { Link, NavLink, Outlet, useLocation } from "react-router-dom";
+import { Link, NavLink, Outlet, useLocation, useNavigationType } from "react-router-dom";
 
 import { CommandPalette } from "@/components/CommandPalette";
 import { StatusFooter } from "@/components/StatusFooter";
@@ -63,6 +63,13 @@ export function AppShell() {
   const showSidebarToggle =
     location.pathname === "/" || location.pathname === "/online";
 
+  const navigationType = useNavigationType();
+  useEffect(() => {
+    // New navigations start at the top; history back/forward keeps position.
+    if (navigationType === "POP") return;
+    window.scrollTo({ top: 0 });
+  }, [location.pathname, navigationType]);
+
   return (
     <div className="flex min-h-screen flex-col">
       <header className="sticky top-0 z-40 border-b border-ctp-surface0 bg-ctp-crust/95 backdrop-blur">