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/calendar_screen.dart

@@ -1,5 +1,6 @@
 // lib/screens/calendar_screen.dart
 import 'package:flutter/material.dart';
+import '../core/utils/error_utils.dart';
 import 'package:intl/intl.dart';
 import 'package:cached_network_image/cached_network_image.dart';
 import '../features/events/services/events_service.dart';
@@ -94,7 +95,7 @@ class _CalendarScreenState extends State<CalendarScreen> {
       }
 
     } catch (e) {
-      if (mounted) ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(e.toString())));
+      if (mounted) ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(userFriendlyError(e))));
     } finally {
       if (mounted) setState(() => _loadingMonth = false);
     }
@@ -117,7 +118,7 @@ class _CalendarScreenState extends State<CalendarScreen> {
       final events = await _service.getEventsForDate(yyyyMMdd);
       if (mounted) setState(() => _eventsOfDay = events);
     } catch (e) {
-      if (mounted) ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(e.toString())));
+      if (mounted) ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(userFriendlyError(e))));
     } finally {
       if (mounted) setState(() => _loadingDay = false);
     }

lib/screens/contribute_screen.dart

@@ -5,6 +5,7 @@
 import 'dart:io';
 import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
+import '../core/utils/error_utils.dart';
 import 'package:flutter/services.dart';
 import 'package:image_picker/image_picker.dart';
 import 'package:provider/provider.dart';
@@ -1829,7 +1830,7 @@ class _ContributeScreenState extends State<ContributeScreen>
       }
     } catch (e) {
       if (mounted) {
-        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text('Failed to pick images: $e')));
+        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(userFriendlyError(e))));
       }
     }
   }
@@ -1868,7 +1869,7 @@ class _ContributeScreenState extends State<ContributeScreen>
       }
     } catch (e) {
       if (mounted) {
-        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text('Error: $e'), backgroundColor: Colors.red));
+        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(userFriendlyError(e)), backgroundColor: Colors.red));
       }
     } finally {
       if (mounted) setState(() => _submitting = false);
@@ -2652,7 +2653,7 @@ class _ContributeScreenState extends State<ContributeScreen>
       );
     } catch (e) {
       if (mounted) {
-        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text('Redemption failed: $e'), backgroundColor: Colors.red));
+        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(userFriendlyError(e)), backgroundColor: Colors.red));
       }
     }
   }

lib/screens/desktop_login_screen.dart

@@ -1,6 +1,7 @@
 // lib/screens/desktop_login_screen.dart
 
 import 'package:flutter/material.dart';
+import '../core/utils/error_utils.dart';
 import '../features/auth/services/auth_service.dart';
 import '../core/auth/auth_guard.dart';
 import 'home_desktop_screen.dart';
@@ -101,7 +102,7 @@ class _DesktopLoginScreenState extends State<DesktopLoginScreen> with SingleTick
       ));
     } catch (e) {
       if (!mounted) return;
-      final message = e.toString().replaceAll('Exception: ', '');
+      final message = userFriendlyError(e);
       ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(message)));
       setState(() => _isAnimating = false);
     } finally {
@@ -335,7 +336,7 @@ class _DesktopRegisterScreenState extends State<DesktopRegisterScreen> {
       Navigator.of(context).pushReplacement(MaterialPageRoute(builder: (_) => const HomeDesktopScreen(skipSidebarEntranceAnimation: true)));
     } catch (e) {
       if (!mounted) return;
-      final message = e.toString().replaceAll('Exception: ', '');
+      final message = userFriendlyError(e);
       ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(message)));
     } finally {
       if (mounted) setState(() => _loading = false);

lib/screens/home_screen.dart

@@ -1,6 +1,7 @@
 // lib/screens/home_screen.dart
 import 'dart:async';
 import 'dart:ui';
+import '../core/utils/error_utils.dart';
 import 'package:flutter/material.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 import '../core/auth/auth_guard.dart';
@@ -122,7 +123,7 @@ class _HomeScreenState extends State<HomeScreen> with SingleTickerProviderStateM
     } catch (e) {
       if (mounted) {
         setState(() => _loading = false);
-        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(e.toString())));
+        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(userFriendlyError(e))));
       }
     }
   }

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);
     }

lib/screens/login_screen.dart

@@ -1,6 +1,7 @@
 // lib/screens/login_screen.dart
 
 import 'dart:ui';
+import '../core/utils/error_utils.dart';
 import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/material.dart';
 import 'package:video_player/video_player.dart';
@@ -106,7 +107,7 @@ class _LoginScreenState extends State<LoginScreen> {
       ));
     } catch (e) {
       if (!mounted) return;
-      final message = e.toString().replaceAll('Exception: ', '');
+      final message = userFriendlyError(e);
       ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(message)));
     } finally {
       if (mounted) setState(() => _loading = false);
@@ -606,7 +607,7 @@ class _RegisterScreenState extends State<RegisterScreen> {
       Navigator.of(context).pushReplacement(MaterialPageRoute(builder: (_) => const HomeScreen()));
     } catch (e) {
       if (!mounted) return;
-      final message = e.toString().replaceAll('Exception: ', '');
+      final message = userFriendlyError(e);
       ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(message)));
     } finally {
       if (mounted) setState(() => _loading = false);

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))));
     }
   }
 

lib/screens/search_screen.dart

@@ -1,6 +1,7 @@
 // lib/screens/search_screen.dart
 import 'dart:ui';
 import 'package:flutter/material.dart';
+import '../core/utils/error_utils.dart';
 
 // Location packages
 import 'package:geolocator/geolocator.dart';
@@ -152,7 +153,7 @@ class _SearchScreenState extends State<SearchScreen> {
       if (mounted) Navigator.of(context).pop('Current Location');
     } catch (e) {
       if (mounted) {
-        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text('Could not determine location: $e')));
+        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(userFriendlyError(e))));
         Navigator.of(context).pop('Current Location');
       }
     } finally {