fix(cli): trim leading slash in download_prefix to prevent Path::join treating relative path as absolute

[ForestRealms]

Summary

Fix rc cp -r incorrectly writing files to the system root (/) when the source prefix lacks a trailing slash (e.g., alias/bucket/dir instead of alias/bucket/dir/).

Related: #244

What Changed

• Modified download_prefix in crates/cli/src/commands/cp.rs:
  • Added .trim_start_matches('/') to the relative key computed by strip_prefix.
  • This ensures the relative path does not start with a slash, preventing Path::join from interpreting it as an absolute path and discarding the destination directory.

Why

When src.key = "dir" and item.key = "dir/file", strip_prefix returns "/file" (leading slash). Path::join treats any argument starting with / as an absolute path, ignoring the user-specified destination and writing files to the system root. This violates user expectations and is inconsistent with other S3 clients like mcli.

Impact

• Users can now use rc cp -r alias/bucket/dir ./ without needing to append a trailing slash.
• Files are correctly copied to the intended destination directory.
• Existing behavior when source prefix ends with / remains unchanged.

Verification

• cargo fmt --all --check (pass)
• cargo clippy --workspace -- -D warnings (pass)
• cargo test --workspace (pass)
• Manually tested on Linux and Windows 11 with both slash-terminated and non-slash-terminated source paths; files correctly copied to the specified destination.