fix(i18n): Round 2 review — internationalize all hardcoded menu strings

C1: Replace all 23+ hardcoded English strings in app_menus.rs with
    crate::menu_label() calls, including debug-only menu item constants
    replaced at point-of-use (cannot call fn in const context).

I1: Fix inconsistent capitalization in YAML locale files:
    'Disable in-band generators' → 'Disable In-band Generators'.

I2: Fix dock_menu() — both 'New Window' labels now use menu_label.

W1: Rewrite menu_label() in lib.rs to use i18n::TranslationLookup
    instead of string comparison, avoiding edge case where translation
    coincidentally equals the lookup key.

W2: Add comment in set_locale() noting zh-CN locale file not yet shipped.

W3: Add 'Reserved for security-sensitive UI per i18n spec' comment on
    t_required! macro.

Author: ErshovDmitry