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/widgets/tier_avatar_ring.dart

@@ -55,6 +55,10 @@ class TierAvatarRing extends StatelessWidget {
 
     return CachedNetworkImage(
       imageUrl: _avatarUrl,
+      memCacheWidth: (size * 2).round(),
+      memCacheHeight: (size * 2).round(),
+      maxWidthDiskCache: (size * 4).round(),
+      maxHeightDiskCache: (size * 4).round(),
       imageBuilder: (context, imageProvider) => CircleAvatar(
         radius: radius,
         backgroundImage: imageProvider,