Drain suppressed binary responses before copying to clipboard

[ryanfowler]

Summary

• Drain binary responses to io.Discard when terminal output is suppressed and --copy is active, so the tee captures the full body instead of only the sniffed prefix.
• Keep the existing binary warning behavior for terminal output.
• Add a regression test that verifies clipboard capture sees the full response body when binary output is suppressed.

Testing

• go test -v ./internal/fetch
• go test -v ./...