fix: show conversations reactions on individual post pages

Conversations items (from Mastodon/Bluesky/ActivityPub) were filtered
out by the client-side timestamp check that prevents duplicating
build-time webmentions. Since conversations data is never in the
build-time cache, bypass the filter for items with a platform field.

js/webmentions.js

@@ -57,8 +57,11 @@
 
     let mentionsToShow;
     if (hasBuildTimeSection) {
-      // Build-time section exists - only show NEW webmentions to avoid duplicates
+      // Build-time section exists - only show NEW webmentions to avoid duplicates.
+      // Conversations items (which have a 'platform' field) are never included
+      // in the build-time cache, so always show them regardless of timestamp.
       mentionsToShow = allChildren.filter((wm) => {
+        if (wm.platform) return true;
         const wmTime = new Date(wm['wm-received']).getTime();
         return wmTime > buildTime;
       });