Motivation

• Replace the previous stub that logged "Clipboard API not implemented for Android" with a working implementation to allow setting the system clipboard from Android.
• Ensure thread-safety and proper JNI attachment/detachment when invoked from native threads.

Description

• Added includes for Activity.hpp, jni.h, and <mutex> and implemented ClipboardImpl::setString(const String& text) to interact with the Android Clipboard via JNI.
• Acquire ActivityStates and lock states.mutex, obtain JavaVM/JNIEnv, and attach the current thread to the JVM when necessary using AttachCurrentThread and DetachCurrentThread.
• Convert the sf::String to UTF-16, create jstring label and data, obtain CLIPBOARD_SERVICE from android/content/Context, call getSystemService, create a ClipData via ClipData.newPlainText, and call ClipboardManager.setPrimaryClip.
• Clean up all local JNI references after the call and only detach the thread if it was attached in this function.

Testing

• Performed an Android build of the project using the standard CI-style build (cmake/NDK/ninja) which completed successfully.
• Tested in my project locally

Codex Task