revert(embed): hide embed area when no thumbnail available

Reverts the link icon placeholder for embeds without thumbnails.
Posts with failed embed processing will simply not show an embed
area instead of displaying a placeholder icon.

Co-Authored-By: Claude Opus 4.5 <[e-mail]>

lib/widgets/post_card.dart

···
 
   @override
   Widget build(BuildContext context) {
-    // Show placeholder when no thumbnail available
+    // Hide embed area when no thumbnail available
     if (widget.embed.thumb == null) {
-      final placeholderWidget = Container(
-        height: widget.height,
-        decoration: BoxDecoration(
-          color: AppColors.backgroundSecondary,
-          borderRadius: BorderRadius.circular(8),
-          border: Border.all(color: AppColors.border),
-        ),
-        child: const Center(
-          child: Icon(
-            Icons.link,
-            color: AppColors.textSecondary,
-            size: 32,
-          ),
-        ),
-      );
-
-      // Make tappable if handler provided
-      if (widget.onImageTap != null) {
-        return GestureDetector(
-          onTap: widget.onImageTap,
-          child: placeholderWidget,
-        );
-      }
-      return placeholderWidget;
+      return const SizedBox.shrink();
     }
 
     // Build the thumbnail image