fix(og): fall back to thumbnail when declared cover file is missing

WeChat link cards were pointing og:image at mindspace-cover paths that
never landed on disk; serve-time injection now uses thumbnail.png instead.

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

mindspace-public-delivery.mjs

@@ -1,4 +1,5 @@
 import crypto from 'node:crypto';
+import fs from 'node:fs';
 import path from 'node:path';
 
 const INLINE_SCRIPT_PATTERN = /<script\b(?![^>]*\bsrc\b)[^>]*>([\s\S]*?)<\/script>/gi;
@@ -47,6 +48,8 @@ export function decorateMindSpacePublishedHtml({
   html,
   embed = false,
   context,
+  htmlFilePath = '',
+  fileExists = fs.existsSync,
   userAgent = '',
   preparePublicationHtmlForEmbed,
   injectOgTags,
@@ -69,6 +72,8 @@ export function decorateMindSpacePublishedHtml({
       pageUrl: context.pageUrl,
       pageDirUrl: context.pageDirUrl,
       fallbackImageUrl: context.fallbackImageUrl,
+      htmlFilePath,
+      fileExists,
     });
     const wechatShare = !embed && isWechatUserAgent(userAgent || '');
     if (wechatShare) {