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/desktop_topbar.dart

@@ -1,3 +1,4 @@
+import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 
 class DesktopTopBar extends StatelessWidget {
@@ -108,7 +109,11 @@ class DesktopTopBar extends StatelessWidget {
         return CircleAvatar(
           radius: 20,
           backgroundColor: Colors.grey.shade200,
-          backgroundImage: NetworkImage(url),
+          backgroundImage: CachedNetworkImageProvider(
+            url,
+            maxWidth: 80,
+            maxHeight: 80,
+          ),
           onBackgroundImageError: (_, __) {},
         );
       }