fix: download folders through symlink roots

[jay23606]

Summary

• Follow the target when a folder download starts from a symlinked directory, while keeping the alias path in the paths sent to the client.
• Close data and resource fork readers after streaming.
• Add a regression test for a relative symlink target.

Fixes #173

Validation

• go test ./hotline -run TestDownloadFolderHandler -count=1 passed.
• go build ./... passed.
• go vet ./... passed.
• go test ./... has existing Windows failures in upload-resume and path handling; the same failures occur on clean upstream master.
• Race tests could not run because this Windows environment has CGO disabled.

[jhalter]

Thanks for the contribution!