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/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;
   }