French (fr) UI localization (#1224)

Adds a French catalog as a partial locale (missing strings fall back to English, like Russian), registered in the language list, with tests and attribution.

crates/ui-egui/src/prefs.rs

@@ -1584,7 +1584,7 @@ mod tests {
         assert!(control(&mut app, &json!({"open": "Plugins"})).is_err());
         assert!(set(&mut app, &json!({"up_axis": "x"})).is_err());
         assert!(set(&mut app, &json!({"theme": "sepia"})).is_err());
-        assert!(set(&mut app, &json!({"language": "fr"})).is_err());
+        assert!(set(&mut app, &json!({"language": "xx"})).is_err());
     }
 
     #[test]