fix(gopher): serve public files from SFTP storage

[rissrice2105-agent]

Summary

• resolve Gopher public-file selectors from the SFTP service's real /files/public/ root
• keep homepage resolution under /users//public_html
• add a test-first regression proving SFTP uploads are listed and stale legacy files are ignored

Testing

• go test ./internal/gopher -run TestPublicFilesUseSFTPStorageRoot -count=1
• go test ./internal/gopher -count=1
• go vet ./internal/gopher
• git diff --check

The full go test ./... -count=1 run passes all relevant packages. Two pre-existing Windows-only tests remain incompatible with this environment: internal/ircpass executes a .sh helper directly, and internal/sites requires symlink privileges.

Closes#114