Refactor async server joining with eJoinState enum and dedicated progress UI

Replace the boolean-flag-based async join system with a clean state machine
(eJoinState enum) and move connection progress handling from UIScene_JoinMenu
into UIScene_ConnectingProgress as a dedicated UI class.

Combines the best of two approaches: non-blocking sockets with select()
timeout and SO_RCVTIMEO clearing (prevents random disconnects) with the
upstream's state enum, FinalizeJoin separation, and ConnectingProgress UI.

JoinGame() now returns JOINGAME_PENDING on Win64, and
PlatformNetworkManagerStub::DoWork() polls the join state to finalize
the connection when the background thread succeeds.

Minecraft.Client/Common/UI/UIScene_ConnectingProgress.h

@@ -12,6 +12,10 @@ private:
 	bool m_showingButton;
 	void (*m_cancelFunc)(LPVOID param);
 	LPVOID m_cancelFuncParam;
+#ifdef _WINDOWS64
+	bool m_asyncJoinActive;
+	bool m_asyncJoinFailed;
+#endif
 
 	enum EControls
 	{