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/profile_screen.dart

@@ -15,6 +15,7 @@ import 'package:shared_preferences/shared_preferences.dart';
 import 'package:provider/provider.dart';
 import '../features/events/services/events_service.dart';
 import '../features/events/models/event_models.dart';
+import '../core/auth/auth_guard.dart';
 import '../features/gamification/providers/gamification_provider.dart';
 import '../features/gamification/models/gamification_models.dart';
 import '../widgets/skeleton_loader.dart';
@@ -123,9 +124,11 @@ class _ProfileScreenState extends State<ProfileScreen>
     _loadProfile();
     _startAnimations();
 
-    // Load gamification data for profile EP cards
+    // Load gamification data for profile EP cards — skip for guests (endpoints authed).
     WidgetsBinding.instance.addPostFrameCallback((_) {
-      if (mounted) context.read<GamificationProvider>().loadAll(force: true);
+      if (mounted && !AuthGuard.isGuest) {
+        context.read<GamificationProvider>().loadAll(force: true);
+      }
     });
   }
 
@@ -1047,6 +1050,8 @@ class _ProfileScreenState extends State<ProfileScreen>
                   imageUrl: imageUrl,
                   memCacheWidth: 120,
                   memCacheHeight: 120,
+                  maxWidthDiskCache: 240,
+                  maxHeightDiskCache: 240,
                   width: 60,
                   height: 60,
                   fit: BoxFit.cover,
@@ -2552,6 +2557,8 @@ class _ProfileScreenState extends State<ProfileScreen>
           imageUrl: imageUrl,
           memCacheWidth: 400,
           memCacheHeight: 400,
+          maxWidthDiskCache: 800,
+          maxHeightDiskCache: 800,
           width: double.infinity,
           height: double.infinity,
           fit: BoxFit.cover,