Add password support and recursive folder download for shared links

[AndreyVMarkelov]

Summary

• Add --password, --password-prompt, --password-file flags to share-link create, share-link update, and share-link download
• Add --remove-password flag to share-link update
• Add --recursive (-r) flag to share-link download for folder shared links
• Password prompt uses golang.org/x/term for secure non-echoing terminal input
• RemoveSharedLinkPassword uses raw SDK request to send require_password: false (not exposed by the Go SDK's ModifySharedLinkSettings)
• Folder download uses BFS traversal, files.ListFolder with shared link context, and per-file retry
• Path traversal protection rejects .., empty components, and absolute paths

Depends on #257.

Test plan

• --password sets password on create, update, and download
• --password-prompt reads from terminal/stdin without echo
• --password-file reads and trims trailing newlines
• Multiple password sources rejected
• Empty password rejected
• --remove-password sends require_password: false via raw API call
• --password + --remove-password mutually exclusive on update
• Folder download requires --recursive flag
• Folder download rejects stdout target
• Recursive download creates nested directories and downloads files
• Folder listing follows pagination
• Target directory reuse appends folder name
• Path traversal attacks rejected
• HTTP-level test verifies RemoveSharedLinkPassword JSON payload
• All flags registered on commands
• golangci-lint clean, all tests pass