fix: avoid keyboard probe for piped stdin

[atirna]

Summary

Skip the keyboard enhancement probe when standard input is piped.

Verification

• before: git diff | tuicr kept running and consumed a CPU core
• after: git diff | tuicr exits with a terminal-input error instead of spinning
• cargo fmt --check
• cargo build
• cargo test
• cargo clippy -- -D warnings

Fixes #675

[agavra]

Thank you @atirna for the contribution! This looks good to me.