fix(files): honor forwarded HTTPS proxy chains

[rissrice2105-agent]

Summary

• parse the first X-Forwarded-Proto value when requests pass through multiple proxies
• keep file-manager session cookies Secure when the client-facing connection is HTTPS
• ignore later forwarded values so an initial http value cannot be upgraded

Testing

• go test ./internal/files
• go vet ./internal/files
• git diff --check