fix: bail when stdout is not a terminal

[Mathjk]

Fixes #601.

Bug

gping <host> | cat (or any piped/redirected stdout) enters the ratatui render loop anyway and floods the consumer with ANSI frames forever. main() never checks stdout; CrosstermBackend happily renders into a pipe — on Windows, enable_raw_mode() only affects the input console, which is still a tty, so nothing stops it.

Fix (+8/-0)

Bail early when stdout isn't a terminal — right after the no-hosts check, before DNS resolution and thread spawn:

if !io::stdout().is_terminal() {
    bail!("gping requires a terminal to draw its graph, but stdout is piped or redirected");
}

Matches top/htop/bottom behavior — gping has no plain-text mode, so refusing is the correct semantics.

Verified (Windows 11 + Linux)

Case	Before	After
gping 127.0.0.1 > out.txt	file grows ~600B/s with ANSI frames forever	clean error on stderr, exit 1, 0 bytes
gping 127.0.0.1 | exit-early	cryptic Error: os error 232	clean error, exit 1
TTY launch under ConPTY	—	renders, q exits cleanly
--help/-V piped	works	unchanged (clap exits during parse)

cargo fmt --check + clippy --workspace clean; cargo test -p gping 18/18 (one pre-existing IPv6 env failure in pinger unrelated to this diff).