fix: add fallback for breadcrumb name to fix SEO validation error

Google Search Console reported "Either name or item.name should be specified"
for the last breadcrumb item. This occurred because post.title can be undefined,
and JSON.stringify omits undefined values.

Added fallback chain (post.title || post.sharedPost?.title || 'Post') and
removed item property from last breadcrumb per Google's recommendation.

Author: nimrodkra

packages/webapp/components/PostSEOSchema.tsx

@@ -149,8 +149,7 @@ export const getBreadcrumbJsonLd = (post: Post): string => {
       {
         '@type': 'ListItem',
         position: 3,
-        name: post.title,
-        item: post.commentsPermalink,
+        name: post.title || post.sharedPost?.title || 'Post',
       },
     ],
   });