style(post): make community avatars circular

Update community avatar styling in post cards from rounded squares
to circles for visual consistency with other avatar elements in the app.

Changes:
- Increase avatar border radius from 4 to 12 for circular CachedNetworkImage
- Use BoxShape.circle for fallback letter avatars
- Add const to fallback avatar decoration

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

lib/widgets/post_card.dart

···
     if (community.avatar != null && community.avatar!.isNotEmpty) {
       // Show real community avatar
       return ClipRRect(
-        borderRadius: BorderRadius.circular(4),
+        borderRadius: BorderRadius.circular(12),
         child: CachedNetworkImage(
           imageUrl: community.avatar!,
           width: 24,
···
     return Container(
       width: 24,
       height: 24,
-      decoration: BoxDecoration(
+      decoration: const BoxDecoration(
         color: AppColors.primary,
-        borderRadius: BorderRadius.circular(4),
+        shape: BoxShape.circle,
       ),
       child: Center(
         child: Text(