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/contributor_profile_screen.dart

@@ -2,6 +2,7 @@
 // CTR-004 — Public contributor profile page.
 // Shows avatar, tier ring, EP stats, and submission grid for any contributor.
 
+import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 
 import '../features/gamification/models/gamification_models.dart';
@@ -215,10 +216,15 @@ class _ContributorProfileScreenState extends State<ContributorProfileScreen> {
             ClipRRect(
               borderRadius: BorderRadius.circular(10),
               child: SizedBox.expand(
-                child: Image.network(
-                  firstImage,
+                child: CachedNetworkImage(
+                  imageUrl: firstImage,
                   fit: BoxFit.cover,
-                  errorBuilder: (_, __, ___) => Container(color: const Color(0xFF334155)),
+                  memCacheWidth: 400,
+                  memCacheHeight: 300,
+                  maxWidthDiskCache: 800,
+                  maxHeightDiskCache: 600,
+                  placeholder: (_, __) => Container(color: const Color(0xFF1E293B)),
+                  errorWidget: (_, __, ___) => Container(color: const Color(0xFF334155)),
                 ),
               ),
             ),