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/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));
       }
     }
   }