feat: add Windows compatibility for external process execution

This PR introduces cross-platform support for launching external binaries (`ace-lm` and `ace-synth`) on Windows systems.

**Key changes:**
* Introduced `EXE_EXT` constant in `AceStepWorker.h` using `Q_OS_WIN` macro to handle `.exe` extensions automatically.
* Updated binary path construction to ensure `QFileInfo::isExecutable()` and `QProcess` work correctly on Windows.
* The code remains fully compatible with Linux/macOS (extension remains empty).

**Why this is needed:**
On Windows, `QProcess` and `QFileInfo` require the `.exe` suffix to correctly identify and execute binary files. Without this change, the application fails to find the required tools even if they are present in the directory.

src/AceStepWorker.cpp

@@ -57,7 +57,7 @@ bool AceStep::requestGeneration(SongItem song, QString requestTemplate, QString
 
 	request = {song, QRandomGenerator::global()->generate(), aceStepPath, textEncoderModelPath, ditModelPath, vaeModelPath};
 
-	QString qwen3Binary = aceStepPath + "/ace-lm";
+	QString qwen3Binary = aceStepPath + "/ace-lm" + EXE_EXT;
 	QFileInfo qwen3Info(qwen3Binary);
 	if (!qwen3Info.exists() || !qwen3Info.isExecutable())
 	{
@@ -141,7 +141,7 @@ void AceStep::qwenProcFinished(int code, QProcess::ExitStatus status)
 		return;
 	}
 
-	QString ditVaeBinary = request.aceStepPath + "/ace-synth";
+	QString ditVaeBinary = request.aceStepPath + "/ace-synth" + EXE_EXT;
 	QFileInfo ditVaeInfo(ditVaeBinary);
 	if (!ditVaeInfo.exists() || !ditVaeInfo.isExecutable())
 	{

src/AceStepWorker.h

@@ -13,6 +13,12 @@
 
 #include "SongItem.h"
 
+#ifdef Q_OS_WIN
+inline const QString EXE_EXT = ".exe";
+#else
+inline const QString EXE_EXT = "";
+#endif
+
 class AceStep : public QObject
 {
 	Q_OBJECT