Render thumb #id as an inline post thumbnail in DText

- New PostThumb component fetches the post through the e621 client
  (cached 30 minutes, no retries) and renders its preview image linked
  to the in-app post page, with the post id underneath
- Falls back to an external e621 link while loading or when the post
  cannot be fetched

frontend/src/features/online/PostThumb.tsx

@@ -0,0 +1,53 @@
+import { useQuery } from "@tanstack/react-query";
+import { Link } from "react-router-dom";
+
+import { effectiveCredentials, fetchPost } from "@/lib/e621";
+import { useE621 } from "@/store/e621";
+
+export function PostThumb({ postId }: { postId: number }) {
+  const credentials = useE621((state) => state.credentials);
+
+  const query = useQuery({
+    queryKey: ["e621", "post-thumb", postId, credentials?.username ?? ""],
+    queryFn: ({ signal }) =>
+      fetchPost(effectiveCredentials(credentials), postId, signal),
+    staleTime: 30 * 60_000,
+    gcTime: 60 * 60_000,
+    retry: 0,
+  });
+
+  const preview = query.data?.preview.url;
+
+  if (preview) {
+    return (
+      <span className="my-1 inline-block align-top">
+        <Link
+          to={`/online/view/${postId}`}
+          title={`Post #${postId}`}
+          className="group inline-block"
+        >
+          <img
+            src={preview}
+            alt={`Post #${postId}`}
+            loading="lazy"
+            className="max-h-40 rounded border border-ctp-surface0 transition group-hover:border-ctp-surface2"
+          />
+          <span className="mt-0.5 block text-center font-mono text-[10px] text-ctp-overlay0">
+            #{postId}
+          </span>
+        </Link>
+      </span>
+    );
+  }
+
+  return (
+    <a
+      href={`https://e621.net/posts/${postId}`}
+      target="_blank"
+      rel="noreferrer"
+      className="text-ctp-blue hover:underline"
+    >
+      thumb #{postId}
+    </a>
+  );
+}

frontend/src/lib/dtext.tsx

@@ -1,6 +1,7 @@
 import type { ReactNode } from "react";
 
 import { Spoiler } from "@/components/Spoiler";
+import { PostThumb } from "@/features/online/PostThumb";
 
 export const DEFAULT_E621_BASE_URL = "https://e621.net";
 
@@ -51,7 +52,6 @@ const ENTITY_PATHS: Record<string, string> = {
   takedown: "/takedowns/{id}",
   record: "/user_feedbacks/{id}",
   ticket: "/tickets/{id}",
-  thumb: "/posts/{id}",
 };
 
 const LINK_RE = new RegExp(
@@ -159,11 +159,13 @@ function linkify(text: string, keyPrefix: string, baseUrl: string): ReactNode[]
     } else if (match[11] !== undefined) {
       nodes.push(<span key={key} id={match[11]} />);
     } else if (match[12] !== undefined && match[13] !== undefined) {
-      const path = ENTITY_PATHS[match[12]]?.replace("{id}", match[13]);
-      const label = `${match[12]} #${match[13]}`;
-      nodes.push(
-        path ? anchor(key, `${baseUrl}${path}`, label) : label,
-      );
+      if (match[12] === "thumb") {
+        nodes.push(<PostThumb key={key} postId={Number(match[13])} />);
+      } else {
+        const path = ENTITY_PATHS[match[12]]?.replace("{id}", match[13]);
+        const label = `${match[12]} #${match[13]}`;
+        nodes.push(path ? anchor(key, `${baseUrl}${path}`, label) : label);
+      }
     }
     lastIndex = regex.lastIndex;
   }