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);
     }