Add share-link download command

[AndreyVMarkelov]

Summary

• Add share-link download <url> [target] to download a file from a Dropbox shared link
• If target is omitted, uses the filename from shared-link metadata
• Use - as target to write file bytes to stdout (byte-clean: progress/status goes to stderr)
• --password flag for password-protected shared links
• Reuses existing download infrastructure (retry, atomic temp-file writes, progress bar, broken pipe handling)
• Rejects folder links and unsafe metadata names

Test plan

• Requires URL argument, rejects empty URL and empty target
• Downloads to metadata filename when target omitted
• Downloads to explicit target path
• Downloads into target directory (appends metadata filename)
• Stdout mode is byte-clean (no progress mixed in)
• No retry after partial stdout writes
• Broken pipe returns nil (no error)
• API errors returned unchanged
• Rejects missing content body
• Rejects folder links (non-file metadata)
• Rejects unsafe metadata names (..)
• Password flag passed to API
• Verbose mode writes status to stderr
• Command registration doesn't break other share-link subcommands
• golangci-lint clean, all tests pass