fix(utils): centralize cron expression validation (closes #863)

[belnoinja]

Closes #863

Cron expression validation was duplicated in two private functions:

• validateCronExpression in pkg/views/scan-all/update/view.go
• validateCron in cmd/harbor/root/scan_all/update_schedule.go

This PR centralizes the logic into a single reusable utils.ValidateCronExpression(expr string) error function in pkg/utils/cron.go and replaces both inline implementations with calls to it.

Changes

• pkg/utils/cron.go — new file with ValidateCronExpression, mirrors exact existing regex and field-count behavior, no new dependencies
• pkg/utils/cron_test.go — 14 table-driven tests (valid expressions, empty input, wrong field counts, out-of-range values)
• pkg/views/scan-all/update/view.go — removed private validateCronExpression, passes utils.ValidateCronExpression directly to huh form Validate callback
• cmd/harbor/root/scan_all/update_schedule.go — removed private validateCron and unused errors import, delegates to utils.ValidateCronExpression

Testing

go build ./...   # exit 0
go test ./...    # all tests pass
go vet ./...     # exit 0