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/linux/my_application.h

@@ -0,0 +1,18 @@
+#ifndef FLUTTER_MY_APPLICATION_H_
+#define FLUTTER_MY_APPLICATION_H_
+
+#include <gtk/gtk.h>
+
+G_DECLARE_FINAL_TYPE(MyApplication, my_application, MY, APPLICATION,
+                     GtkApplication)
+
+/**
+ * my_application_new:
+ *
+ * Creates a new Flutter-based application.
+ *
+ * Returns: a new #MyApplication.
+ */
+MyApplication* my_application_new();
+
+#endif  // FLUTTER_MY_APPLICATION_H_