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/profile_screen.dart

@@ -1,6 +1,7 @@
 import 'dart:io';
 
 import 'package:cached_network_image/cached_network_image.dart';
+import '../core/utils/error_utils.dart';
 import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:image_picker/image_picker.dart';
@@ -211,7 +212,7 @@ class _ProfileScreenState extends State<ProfileScreen>
     } catch (e) {
       if (mounted) {
         ScaffoldMessenger.of(context)
-            .showSnackBar(SnackBar(content: Text('Failed to load events: $e')));
+            .showSnackBar(SnackBar(content: Text(userFriendlyError(e))));
       }
     } finally {
       if (mounted) setState(() => _loadingEvents = false);
@@ -259,7 +260,7 @@ class _ProfileScreenState extends State<ProfileScreen>
     } catch (e) {
       debugPrint('Image pick error: $e');
       ScaffoldMessenger.of(context)
-          .showSnackBar(SnackBar(content: Text('Failed to pick image: $e')));
+          .showSnackBar(SnackBar(content: Text(userFriendlyError(e))));
     }
   }