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/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);