i18n: add Polish (pl) interface language

Polish menus (every fixed menu entry, 630 rows) and panels (every row of
the Chinese panel catalogs, 695), the undo/redo, Reveal and Assign Shortcut
labels, the macOS Services item, Settings ▸ General ▸ Language and
Match System for pl-* locales. Tests cover the locale mapping, the
settings registry and the macOS application menu.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DukHNvUY45rjRnvtz91x33

crates/engine/src/settings/prefs/choices.rs

@@ -8,6 +8,7 @@ pub const LANGUAGES: &[(&str, &str)] = &[
     ("Deutsch", "de"),
     ("Español", "es"),
     ("Français", "fr"),
+    ("Polski", "pl"),
     ("Português (Brasil)", "pt-br"),
     ("Русский", "ru"),
     ("Українська", "uk"),