perf: fix image loading performance across all screens

- Replace Image.network (no cache) with CachedNetworkImage in contributor_profile_screen
- Replace NetworkImage (no cache) with CachedNetworkImageProvider in desktop_topbar and contribute_screen (leaderboard avatars)
- Add maxWidthDiskCache + maxHeightDiskCache to all 23 CachedNetworkImage calls
- Add missing memCacheWidth/Height to review_card (36x36 avatar) and learn_more related events (140x100)
- Add dynamic memCache sizing to tier_avatar_ring based on widget size

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/features/reviews/widgets/review_card.dart

@@ -125,6 +125,10 @@ class _ReviewCardState extends State<ReviewCard> {
                   imageUrl: 'https://api.dicebear.com/9.x/notionists/svg?seed=${Uri.encodeComponent(_review.username)}',
                   width: 36,
                   height: 36,
+                  memCacheWidth: 72,
+                  memCacheHeight: 72,
+                  maxWidthDiskCache: 144,
+                  maxHeightDiskCache: 144,
                   placeholder: (_, __) => CircleAvatar(
                     radius: 18,
                     backgroundColor: _avatarColor(_review.username),