feat: add detailed error display widget for debugging

Problem:
- Gray screens without error messages made debugging impossible
- Users couldn't see what went wrong
- Developers couldn't debug issues without full stack traces

Solution:

1. Created ErrorDisplay widget (lib/presentation/widgets/error_display.dart):
   ✅ Shows detailed error message with copy button
   ✅ Expandable stack trace section with syntax highlighting
   ✅ Retry button for failed operations
   ✅ Debug tips for troubleshooting
   ✅ Beautiful UI with icons, colors, and proper styling
   ✅ Fully selectable text for easy copying

   Features:
   - 🔴 Red error card with full error message
   - 🟠 Orange expandable stack trace panel
   - 🔵 Blue tips panel with debugging suggestions
   - 📋 Copy buttons for error and stack trace
   - 🔄 Retry button to attempt operation again
   - 📱 Responsive scrolling for long errors

2. Updated MovieDetailProvider:
   ✅ Added _stackTrace field to store full stack trace
   ✅ Save stack trace in catch block: catch (e, stackTrace)
   ✅ Expose via getter: String? get stackTrace

3. Updated DownloadsProvider:
   ✅ Added _stackTrace field
   ✅ Updated _setError() to accept optional stackTrace parameter
   ✅ Save stack trace in refreshDownloads() catch block
   ✅ Print error and stack trace to console

4. Updated MovieDetailScreen:
   ✅ Replaced simple Text('Error: ...') with ErrorDisplay widget
   ✅ Shows 'Ошибка загрузки фильма/сериала' title
   ✅ Pass error, stackTrace, and onRetry callback
   ✅ Retry attempts to reload media

5. Updated DownloadsScreen:
   ✅ Replaced custom error UI with ErrorDisplay widget
   ✅ Shows 'Ошибка загрузки торрентов' title
   ✅ Pass error, stackTrace, and onRetry callback
   ✅ Retry attempts to refresh downloads

Error Display Features:
----------------------------
📋 Сообщение об ошибке:
   [Red card with full error text]
   [Copy button]

🐛 Stack Trace (для разработчиков):
   [Expandable orange section]
   [Black terminal-style with green text]
   [Copy stack trace button]

💡 Советы по отладке:
   • Скопируйте ошибку и отправьте разработчику
   • Проверьте соединение с интернетом
   • Проверьте логи Flutter в консоли
   • Попробуйте перезапустить приложение

🔄 [Попробовать снова] button

Example Error Display:
----------------------
┌────────────────────────────────────┐
│        ⚠️  Произошла ошибка        │
│                                    │
│  📋 Сообщение об ошибке:           │
│  ┌──────────────────────────────┐ │
│  │ Exception: Failed to load    │ │
│  │ movie: 404 - Not Found       │ │
│  │ [Копировать ошибку]          │ │
│  └──────────────────────────────┘ │
│                                    │
│  🐛 Stack Trace ▶                  │
│                                    │
│  💡 Советы по отладке:             │
│  ┌──────────────────────────────┐ │
│  │ • Скопируйте ошибку...       │ │
│  │ • Проверьте соединение...    │ │
│  └──────────────────────────────┘ │
│                                    │
│      [🔄 Попробовать снова]        │
└────────────────────────────────────┘

Changes:
- lib/presentation/widgets/error_display.dart (NEW): 254 lines
- lib/presentation/providers/movie_detail_provider.dart: +4 lines
- lib/presentation/providers/downloads_provider.dart: +6 lines
- lib/presentation/screens/movie_detail/movie_detail_screen.dart: +11/-2 lines
- lib/presentation/screens/downloads/downloads_screen.dart: +4/-27 lines

Result:
✅ No more gray screens without explanation!
✅ Full error messages visible on screen
✅ Stack traces available for developers
✅ Copy button for easy error reporting
✅ Retry button for quick recovery
✅ Beautiful, user-friendly error UI
✅ Much easier debugging process

Testing:
--------
1. Open app and tap on a movie card
2. If error occurs, you'll see:
   - Full error message in red card
   - Stack trace in expandable section
   - Copy buttons for error and stack trace
   - Retry button to try again
3. Same for Downloads screen

Now debugging is 10x easier! 🎉

lib/presentation/screens/downloads/downloads_screen.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import '../../providers/downloads_provider.dart';
+import '../../widgets/error_display.dart';
 import '../../../data/models/torrent_info.dart';
 import 'torrent_detail_screen.dart';
 
@@ -46,37 +47,13 @@ class _DownloadsScreenState extends State<DownloadsScreen> {
           }
 
           if (provider.error != null) {
-            return Center(
-              child: Column(
-                mainAxisAlignment: MainAxisAlignment.center,
-                children: [
-                  Icon(
-                    Icons.error_outline,
-                    size: 64,
-                    color: Colors.red.shade300,
-                  ),
-                  const SizedBox(height: 16),
-                  Text(
-                    'Ошибка загрузки',
-                    style: Theme.of(context).textTheme.headlineSmall,
-                  ),
-                  const SizedBox(height: 8),
-                  Text(
-                    provider.error!,
-                    textAlign: TextAlign.center,
-                    style: Theme.of(context).textTheme.bodyMedium?.copyWith(
-                      color: Colors.grey.shade600,
-                    ),
-                  ),
-                  const SizedBox(height: 16),
-                  ElevatedButton(
-                    onPressed: () {
-                      provider.refreshDownloads();
-                    },
-                    child: const Text('Попробовать снова'),
-                  ),
-                ],
-              ),
+            return ErrorDisplay(
+              title: 'Ошибка загрузки торрентов',
+              error: provider.error!,
+              stackTrace: provider.stackTrace,
+              onRetry: () {
+                provider.refreshDownloads();
+              },
             );
           }