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_screen.dart

@@ -2,6 +2,7 @@
 import 'dart:async';
 import 'dart:ui';
 import '../core/utils/error_utils.dart';
+import 'package:flutter/foundation.dart' show kDebugMode, debugPrint;
 import 'package:flutter/material.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 import '../core/auth/auth_guard.dart';
@@ -136,15 +137,16 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
           _loading = false;
         });
       }
-    } catch (e) {
+    } catch (e, st) {
+      if (kDebugMode) debugPrint('HomeScreen._loadUserDataAndEvents error: $e\n$st');
       if (mounted) {
         setState(() => _loading = false);
         ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(userFriendlyError(e))));
       }
     }
 
-    // Refresh notification badge count (fire-and-forget)
-    if (mounted) {
+    // Refresh notification badge count (fire-and-forget, skip for guests — endpoint is authed)
+    if (mounted && !AuthGuard.isGuest) {
       context.read<NotificationProvider>().refreshUnreadCount();
     }
   }
@@ -263,6 +265,8 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
                         imageUrl: imageUrl,
                         memCacheWidth: 112,
                         memCacheHeight: 112,
+                        maxWidthDiskCache: 224,
+                        maxHeightDiskCache: 224,
                         fit: BoxFit.contain,
                         placeholder: (_, __) => Icon(
                           icon ?? Icons.category,
@@ -475,6 +479,8 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
                                           imageUrl: img,
                                           memCacheWidth: 112,
                                           memCacheHeight: 112,
+                                          maxWidthDiskCache: 224,
+                                          maxHeightDiskCache: 224,
                                           width: 56,
                                           height: 56,
                                           fit: BoxFit.cover,
@@ -956,6 +962,8 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
           imageUrl: imageUrl,
           memCacheWidth: 160,
           memCacheHeight: 160,
+          maxWidthDiskCache: 320,
+          maxHeightDiskCache: 320,
           width: 80,
           height: 80,
           fit: BoxFit.cover,
@@ -1288,6 +1296,8 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
                         imageUrl: currentImg,
                         memCacheWidth: 200,
                         memCacheHeight: 200,
+                        maxWidthDiskCache: 400,
+                        maxHeightDiskCache: 400,
                         fit: BoxFit.cover,
                         placeholder: (_, __) => const SizedBox.shrink(),
                         errorWidget: (_, __, ___) => const SizedBox.shrink(),
@@ -1524,6 +1534,8 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
                       imageUrl: img,
                       memCacheWidth: 700,
                       memCacheHeight: 400,
+                      maxWidthDiskCache: 1200,
+                      maxHeightDiskCache: 700,
                       fit: BoxFit.cover,
                       placeholder: (_, __) => const _HeroShimmer(radius: radius),
                       errorWidget: (_, __, ___) =>
@@ -1951,6 +1963,8 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
                     imageUrl: img,
                     memCacheWidth: 300,
                     memCacheHeight: 200,
+                    maxWidthDiskCache: 600,
+                    maxHeightDiskCache: 400,
                     fit: BoxFit.cover,
                     width: double.infinity,
                     height: double.infinity,
@@ -2147,6 +2161,8 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
                       imageUrl: img,
                       memCacheWidth: 192,
                       memCacheHeight: 192,
+                      maxWidthDiskCache: 384,
+                      maxHeightDiskCache: 384,
                       width: 96,
                       height: double.infinity,
                       fit: BoxFit.cover,
@@ -2214,6 +2230,8 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
                           imageUrl: img,
                           memCacheWidth: 440,
                           memCacheHeight: 360,
+                          maxWidthDiskCache: 880,
+                          maxHeightDiskCache: 720,
                           width: 220,
                           height: 180,
                           fit: BoxFit.cover,
@@ -2387,6 +2405,8 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
                           imageUrl: img,
                           memCacheWidth: 800,
                           memCacheHeight: 400,
+                          maxWidthDiskCache: 1200,
+                          maxHeightDiskCache: 600,
                           width: double.infinity,
                           height: 200,
                           fit: BoxFit.cover,