Add translation support for TUI help groups and binding descriptions

[Softer]

Help group names shown via F1 (General, Navigation, Selection, Search) and key binding descriptions in the Textual footer (Down, Up, Cancel, Confirm, etc.) were hardcoded in English and never went through the translation system.

PR Description:

This adds tr() wrapping for help group titles and introduces a mechanism to translate Textual binding descriptions at runtime. The tricky part is that Textual evaluates bindings at class definition time (before any language is activated), and stores them as frozen dataclasses. To work around this, _translate_bindings() rebuilds binding objects on every change of screen, so now we always show the current language. The app-level bindings are also re-translated when the user switches language mid-session.

New translatable strings are added to base.pot and translated for Ukrainian.

[Torxed]

Haven't tested this, but the code looks good.
Only thing that worry me is that we're calling private functions on the widgets which might be subject to change in the future.
But I also recognize that textual might not have thought of this behavior during translations so, probably best option.