`theme create` exits 0 without creating anything when stdin is piped

[falhenaki]

When stdin isn't a real TTY (piped input, CI runner, any scripted context), theme create prints the banner and the first prompt, then exits immediately:

$ printf '\n' | salla theme create -t twig -n my-theme -y; echo $?
...banner...
? Start a new theme  …
0

No theme is created, no error is printed, and the exit code is 0 — so a script has no way to tell the command failed. If interactive mode genuinely can't run, it should exit non-zero with a clear message.

Related to #129 — if --yes worked as documented there would be no prompt to fail on.

Environment: CLI 3.2.36, macOS (arm64), Node 22.23.2.