fix(cli): gate handle_gui_frame_command behind the gui feature

`--no-default-features --features cli` failed to build: the function names
simonlib::gui and carried no cfg, though its only caller was already behind
one. Nothing broke at runtime, and the cli feature broke completely.

This is the failure the CI feature-isolation job exists to catch — a feature
that only builds because another supplies what it is missing — recurring
after the 5.0.0 GUI restore, so it has been live since then. Found by
running that job locally rather than by CI reporting it.

Verified: all 19 advertised features and the no-feature build compile in
isolation.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>