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/reviews/widgets/review_section.dart

@@ -1,6 +1,7 @@
 // lib/features/reviews/widgets/review_section.dart
 import 'package:flutter/material.dart';
 import '../../../core/storage/token_storage.dart';
+import '../../../core/utils/error_utils.dart';
 import '../models/review_models.dart';
 import '../services/review_service.dart';
 import 'review_summary.dart';
@@ -55,7 +56,7 @@ class _ReviewSectionState extends State<ReviewSection> {
         });
       }
     } catch (e) {
-      if (mounted) setState(() { _loading = false; _error = e.toString(); });
+      if (mounted) setState(() { _loading = false; _error = userFriendlyError(e); });
     }
   }