storage: support Windows

Packages/MobileSupportStorage/Runtime/MobileSupport.asmdef

@@ -5,7 +5,8 @@
     "includePlatforms": [
         "Android",
         "Editor",
-        "iOS"
+        "iOS",
+        "WindowsStandalone64"
     ],
     "excludePlatforms": [],
     "allowUnsafeCode": false,

Packages/MobileSupportStorage/Runtime/Scripts/Storage.Windows.cs

@@ -0,0 +1,39 @@
+// --------------------------------------------------------------
+// Copyright 2026 CyberAgent, Inc.
+// --------------------------------------------------------------
+
+#if UNITY_STANDALONE_WIN
+using System.Runtime.InteropServices;
+using UnityEngine;
+
+namespace MobileSupport
+{
+    public static class Storage
+    {
+        [DllImport("kernel32.dll", CharSet = CharSet.Auto)]
+        [return: MarshalAs(UnmanagedType.Bool)]
+        private static extern bool GetDiskFreeSpaceEx(
+            string lpDirectoryName,
+            out ulong lpFreeBytesAvailable,
+            out ulong lpTotalNumberOfBytes,
+            out ulong lpTotalNumberOfFreeBytes);
+
+        /// <summary>
+        ///     Get usable space of internal storage.
+        /// </summary>
+        /// <returns>Bytes of usable space. It will return -1 for internal error and in Unity editor.</returns>
+        public static long GetInternalUsableSpace()
+        {
+#if UNITY_EDITOR
+            if (Application.isEditor) return -1;
+#endif
+
+            if (GetDiskFreeSpaceEx(Application.persistentDataPath,
+                    out var freeBytesAvailable, out _, out _))
+                return (long)freeBytesAvailable;
+
+            return -1;
+        }
+    }
+}
+#endif