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/screens/learn_more_screen.dart

@@ -12,6 +12,7 @@ import 'package:cached_network_image/cached_network_image.dart';
 import '../features/events/models/event_models.dart';
 import '../features/events/services/events_service.dart';
 import '../core/auth/auth_guard.dart';
+import '../core/utils/error_utils.dart';
 import '../core/constants.dart';
 import '../features/reviews/widgets/review_section.dart';
 
@@ -108,7 +109,7 @@ class _LearnMoreScreenState extends State<LearnMoreScreen> {
       _startAutoScroll();
     } catch (e) {
       if (!mounted) return;
-      setState(() => _error = e.toString());
+      setState(() => _error = userFriendlyError(e));
     } finally {
       if (mounted) setState(() => _loading = false);
     }