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/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);