i18n: add Czech (cs) interface language

Czech for all 618 fixed menu entries and the 496 panel, tooltip and dialog
strings of the most complete catalog; settings accept and persist `cs`, Match
System recognises cs, cs-CZ and cs_CZ.UTF-8; native menu, Undo/Redo and
file-manager labels. Czech may lag the Chinese catalogs like Japanese and
Ukrainian (a missing row falls back to English); menu rows use "..." like
the other catalogs. AI-assisted, independently reviewed, not yet proofread
by a native speaker.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/prefs.rs

@@ -32,6 +32,7 @@ pub const LANGUAGES: &[(&str, &str)] = &[
     ("Українська", "uk"),
     ("Español", "es"),
     ("Português (Brasil)", "pt-br"),
+    ("Čeština", "cs"),
 ];
 
 /// Settings ▸ Appearance ▸ UI Scale: (label, `appearance.uiScale` percent).

crates/engine/src/tests_settings.rs

@@ -615,6 +615,14 @@ fn interface_language_is_validated_persisted_and_backward_compatible() {
     let mut es = Session { config: Some(store.clone()), ..Default::default() };
     es.load_settings();
     assert_eq!(es.prefs.general.language, "es");
+    // Czech uses the same validation, persistence and JSON round-trip as every catalog.
+    assert!(crate::prefs::LANGUAGES.contains(&("Čeština", "cs")));
+    s.execute("prefs.set", json!({"key": "general.language", "value": "cs"})).unwrap();
+    assert_eq!(s.execute("prefs.get", json!({"key": "general.language"})).unwrap(), json!("cs"));
+    let mut cs = Session { config: Some(store.clone()), ..Default::default() };
+    cs.load_settings();
+    assert_eq!(cs.prefs.general.language, "cs");
+    assert_eq!(Prefs::from_json(r#"{"general":{"language":"cs"}}"#).general.language, "cs");
     s.execute("prefs.set", json!({"key": "general.language", "value": "ja"})).unwrap();
     let mut reloaded = Session { config: Some(store), ..Default::default() };
     reloaded.load_settings();