сборка: APK (Android) + Linux Desktop

- Исправлены ошибки Flutter анализа (withOpacity, pbkdf2, импорты)
- Упрощён pubspec.yaml (только нужные зависимости)
- Android APK собран: build/app/outputs/flutter-apk/app-release.apk (22MB)
- Linux Desktop собран: build/linux/x64/release/bundle/jam_client

Для Windows: кросс-компиляция невозможна с Linux.
Нужен Windows хост для flutter build windows.

client/lib/screens/splash_screen.dart

@@ -62,7 +62,7 @@ class _SplashScreenState extends State<SplashScreen>
                       borderRadius: BorderRadius.circular(24),
                       boxShadow: [
                         BoxShadow(
-                          color: ThemeService.primary.withValues(alpha: 0.4),
+                          color: ThemeService.primary.withOpacity(0.4),
                           blurRadius: 30,
                           spreadRadius: 5,
                         ),
@@ -94,7 +94,7 @@ class _SplashScreenState extends State<SplashScreen>
                   Text(
                     'Lightweight · Decentralized · Secure',
                     style: TextStyle(
-                      color: ThemeService.textSecondary.withValues(alpha: 0.7),
+                      color: ThemeService.textSecondary.withOpacity(0.7),
                       fontSize: 14,
                       letterSpacing: 0.5,
                     ),