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_bsd.go

@@ -0,0 +1,15 @@
+// +build openbsd
+
+package restore
+
+import "syscall"
+
+// getDiskSpace returns available disk space in bytes (OpenBSD)
+func getDiskSpace(path string) (int64, error) {
+	var statfs syscall.Statfs_t
+	if err := syscall.Statfs(path, &statfs); err != nil {
+		return 0, err
+	}
+	// OpenBSD uses F_bavail and F_bsize
+	return int64(statfs.F_bavail) * int64(statfs.F_bsize), nil
+}