[feat] make MainWindow and ServerWindow use rounded corners

src/common/rounded_corner_button.h

@@ -0,0 +1,20 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2026-02-26
+ * Copyright (c) 2026 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _ROUNDED_CORNER_BUTTON_H_
+#define _ROUNDED_CORNER_BUTTON_H_
+
+#include "imgui.h"
+#include "imgui_internal.h"
+
+namespace crossdesk {
+bool RoundedCornerButton(const char* label, const ImVec2& size, float rounding,
+                         ImDrawFlags round_flags, bool enabled = true,
+                         ImU32 normal_col = 0, ImU32 hover_col = 0,
+                         ImU32 active_col = 0, ImU32 border_col = 0);
+}  // namespace crossdesk
+
+#endif