Fix cross-platform compilation for all target platforms

- Fixed type mismatch in disk space calculation (int64 casting)
- Created platform-specific disk space implementations:
  * diskspace_unix.go (Linux, macOS, FreeBSD)
  * diskspace_windows.go (Windows)
  * diskspace_bsd.go (OpenBSD)
  * diskspace_netbsd.go (NetBSD fallback)
- All 10 platforms now compile successfully:
  ✅ Linux (amd64, arm64, armv7)
  ✅ macOS (Intel, Apple Silicon)
  ✅ Windows (amd64, arm64)
  ✅ FreeBSD, OpenBSD, NetBSD

internal/restore/diskspace_windows.go

@@ -0,0 +1,38 @@
+// +build windows
+
+package restore
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+var (
+	kernel32         = syscall.NewLazyDLL("kernel32.dll")
+	getDiskFreeSpace = kernel32.NewProc("GetDiskFreeSpaceExW")
+)
+
+// getDiskSpace returns available disk space in bytes (Windows)
+func getDiskSpace(path string) (int64, error) {
+	var freeBytesAvailable int64
+	var totalBytes int64
+	var totalFreeBytes int64
+
+	pathPtr, err := syscall.UTF16PtrFromString(path)
+	if err != nil {
+		return 0, err
+	}
+
+	ret, _, err := getDiskFreeSpace.Call(
+		uintptr(unsafe.Pointer(pathPtr)),
+		uintptr(unsafe.Pointer(&freeBytesAvailable)),
+		uintptr(unsafe.Pointer(&totalBytes)),
+		uintptr(unsafe.Pointer(&totalFreeBytes)),
+	)
+
+	if ret == 0 {
+		return 0, err
+	}
+
+	return freeBytesAvailable, nil
+}