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/safety.go

@@ -8,7 +8,6 @@ import (
 	"os"
 	"os/exec"
 	"strings"
-	"syscall"
 
 	"dbbackup/internal/config"
 	"dbbackup/internal/logger"
@@ -243,14 +242,12 @@ func (s *Safety) CheckDiskSpace(archivePath string, multiplier float64) error {
 	requiredSpace := int64(float64(archiveSize) * multiplier)
 
 	// Get available disk space
-	var statfs syscall.Statfs_t
-	if err := syscall.Statfs(s.cfg.BackupDir, &statfs); err != nil {
+	availableSpace, err := getDiskSpace(s.cfg.BackupDir)
+	if err != nil {
 		s.log.Warn("Cannot check disk space", "error", err)
 		return nil // Don't fail if we can't check
 	}
 
-	availableSpace := int64(statfs.Bavail) * statfs.Bsize
-
 	if availableSpace < requiredSpace {
 		return fmt.Errorf("insufficient disk space: need %s, have %s",
 			FormatBytes(requiredSpace), FormatBytes(availableSpace))