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-public-finish-sync.mjs

@@ -3,6 +3,7 @@ import path from 'node:path';
 
 import { extractStaticPageLinks } from './mindspace-chat-save.mjs';
 import { DOWNLOADABLE_FILE_PATTERN } from './mindspace-html-download-links.mjs';
+import { scheduleWorkspaceHtmlThumbnailSidecars } from './mindspace-workspace-thumbnails.mjs';
 
 /**
  * Public HTML finish-sync invariants (regression guard — do not simplify away).
@@ -398,6 +399,7 @@ export function materializeMissingPublicHtmlWrites({ messages, publishDir }) {
       fs.mkdirSync(path.dirname(destination), { recursive: true });
       fs.writeFileSync(destination, artifact.content, 'utf8');
       materialized.push(artifact.relativePath);
+      scheduleWorkspaceHtmlThumbnailSidecars(root, artifact.relativePath);
     } catch {
       skipped.push(artifact.relativePath);
     }