Add --if-exists fail|skip to cp and mv commands

[AndreyVMarkelov]

Summary

• Add --if-exists fail|skip flag to both cp and mv commands (default: fail)
• Pre-flight GetMetadata check skips destinations that already exist when --if-exists skip
• Post-flight conflict recovery converts destination-conflict API errors to skips (handles races)
• JSON output reports per-item "skipped" status alongside "copied" / "moved"
• Text mode is silent for skipped items

Test plan

• go test ./cmd/... passes
• golangci-lint run clean
• dbxcli cp --if-exists skip /src /existing-dest skips without error
• dbxcli mv --if-exists skip /src /existing-dest skips without error
• dbxcli cp --if-exists fail /src /existing-dest fails (default behavior)
• dbxcli cp --if-exists invalid /src /dest returns validation error
• Multi-source cp --if-exists skip applies per-target independently