security: sanitize all error messages shown to users

Created centralized userFriendlyError() utility that converts raw
exceptions into clean, user-friendly messages. Strips hostnames,
ports, OS error codes, HTTP status codes, stack traces, and Django
field names. Maps network/timeout/auth/server errors to plain
English messages.

Fixed 16 locations across 10 files:
- home_screen, calendar_screen, learn_more_screen (SnackBar/Text)
- login_screen, desktop_login_screen (SnackBar)
- profile_screen, contribute_screen, search_screen (SnackBar)
- review_form, review_section (inline error text)
- gamification_provider (error field)

Also removed double-wrapped exceptions in ReviewService (rethrow
instead of throw Exception('Failed to...: $e')).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/features/gamification/providers/gamification_provider.dart

@@ -1,6 +1,7 @@
 // lib/features/gamification/providers/gamification_provider.dart
 
 import 'package:flutter/foundation.dart';
+import '../../../core/utils/error_utils.dart';
 import '../models/gamification_models.dart';
 import '../services/gamification_service.dart';
 
@@ -41,7 +42,7 @@ class GamificationProvider extends ChangeNotifier {
       shopItems  = results[2] as List<ShopItem>;
       achievements = results[3] as List<AchievementBadge>;
     } catch (e) {
-      error = e.toString();
+      error = userFriendlyError(e);
     } finally {
       isLoading = false;
       notifyListeners();
@@ -58,7 +59,7 @@ class GamificationProvider extends ChangeNotifier {
     try {
       leaderboard = await _service.getLeaderboard(district: district, timePeriod: leaderboardTimePeriod);
     } catch (e) {
-      error = e.toString();
+      error = userFriendlyError(e);
     }
     notifyListeners();
   }
@@ -73,7 +74,7 @@ class GamificationProvider extends ChangeNotifier {
     try {
       leaderboard = await _service.getLeaderboard(district: leaderboardDistrict, timePeriod: period);
     } catch (e) {
-      error = e.toString();
+      error = userFriendlyError(e);
     }
     notifyListeners();
   }