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/calendar_screen.dart

@@ -519,6 +519,8 @@ class _CalendarScreenState extends State<CalendarScreen> {
                       imageUrl: imgUrl,
                       memCacheWidth: 400,
                       memCacheHeight: 300,
+                      maxWidthDiskCache: 800,
+                      maxHeightDiskCache: 600,
                       height: 150,
                       width: double.infinity,
                       fit: BoxFit.cover,
@@ -582,6 +584,8 @@ class _CalendarScreenState extends State<CalendarScreen> {
                       imageUrl: imgUrl,
                       memCacheWidth: 300,
                       memCacheHeight: 300,
+                      maxWidthDiskCache: 600,
+                      maxHeightDiskCache: 600,
                       width: 100,
                       height: 100,
                       fit: BoxFit.cover,