fix: restore public page share thumbnails from SVG sidecars

Serve missing *.thumbnail.png requests when the SVG sidecar exists, and schedule async PNG rasterization after public HTML is materialized so WeChat og:image URLs stop 404ing.

Co-authored-by: Cursor <cursoragent@cursor.com>

mindspace-workspace-thumbnails.mjs

@@ -1,6 +1,7 @@
 import fs from 'node:fs';
 import fsPromises from 'node:fs/promises';
 import path from 'node:path';
+import { ensureThumbnailPng } from './mindspace-thumbnail-png.mjs';
 import { ensureHtmlThumbnail } from './mindspace-thumbnails.mjs';
 
 export function workspaceThumbnailRelativePath(htmlRelativePath) {
@@ -15,14 +16,34 @@ export function workspaceThumbnailAbsolutePath(publishDir, htmlRelativePath) {
   return path.join(publishDir, workspaceThumbnailRelativePath(htmlRelativePath));
 }
 
+function scheduleWorkspaceThumbnailPng(publishDir, htmlRelativePath) {
+  queueMicrotask(() => {
+    void Promise.resolve()
+      .then(() => {
+        const svgPath = workspaceThumbnailAbsolutePath(publishDir, htmlRelativePath);
+        if (fs.existsSync(svgPath)) ensureThumbnailPng(svgPath);
+      })
+      .catch(() => {});
+  });
+}
+
+/** Fire-and-forget SVG + PNG sidecars after public HTML is written (must not block Finish/chat). */
+export function scheduleWorkspaceHtmlThumbnailSidecars(publishDir, htmlRelativePath) {
+  queueMicrotask(() => {
+    void ensureWorkspaceHtmlThumbnail(publishDir, htmlRelativePath).catch(() => {});
+  });
+}
+
 export async function ensureWorkspaceHtmlThumbnail(publishDir, htmlRelativePath, html, meta = {}) {
   const htmlPath = path.join(publishDir, htmlRelativePath);
   const content = html ?? (await fsPromises.readFile(htmlPath, 'utf8'));
   const thumbRel = workspaceThumbnailRelativePath(htmlRelativePath);
-  return ensureHtmlThumbnail(publishDir, thumbRel, content, {
+  const svg = await ensureHtmlThumbnail(publishDir, thumbRel, content, {
     ...meta,
     contentBaseDir: path.dirname(htmlPath),
   });
+  scheduleWorkspaceThumbnailPng(publishDir, htmlRelativePath);
+  return svg;
 }
 
 export async function readWorkspaceThumbnailIfExists(publishDir, htmlRelativePath) {