Adds crates/ui-egui/locales/cs.toml with all 2165 entries (ui, commands,
menu), marked "draft — needs native review" and kept out of COMPLETE.
Plurals use one/other only; the other form is worded count-neutral so it
reads correctly for 2-4 as well. Lists cs in the pref.language help text,
adds a cs_CZ.UTF-8 detection test and updates the localization docs.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
