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/screens/home_desktop_screen.dart

@@ -320,6 +320,8 @@ class _HomeContentState extends State<_HomeContent>
                           height: double.infinity,
                           memCacheWidth: 1400,
                           memCacheHeight: 800,
+                          maxWidthDiskCache: 1400,
+                          maxHeightDiskCache: 800,
                           placeholder: (_, __) => Container(
                             color: const Color(0xFF0A0E1A),
                           ),
@@ -529,6 +531,8 @@ class _HomeContentState extends State<_HomeContent>
                             fit: BoxFit.cover,
                             memCacheWidth: 1400,
                             memCacheHeight: 800,
+                            maxWidthDiskCache: 1400,
+                            maxHeightDiskCache: 800,
                           )
                         else
                           Container(color: const Color(0xFF0A0E1A)),
@@ -782,6 +786,8 @@ class _HomeContentState extends State<_HomeContent>
                             imageUrl: img,
                             memCacheWidth: 600,
                             memCacheHeight: 320,
+                            maxWidthDiskCache: 1200,
+                            maxHeightDiskCache: 640,
                             width: double.infinity,
                             height: imageHeight,
                             fit: BoxFit.cover,