i18n: add Belarusian translation (be) (#12284)

This PR adds Belarusian (be) language support to Ghostty.

## Changes

- `po/be.po` — new Belarusian translation file (80 strings)
- `src/os/i18n_locales.zig` — added `be` locale
- `CODEOWNERS` — added `/po/be.po @ghostty-org/be_BY`

## Notes

Terminology was cross-referenced with:
- KDE Belarusian translations (l10n.kde.org)
- qBittorrent Belarusian translation
- far2l Belarusian translation
- Ubuntu Belarusian Translators Dictionary

src/os/i18n_locales.zig

@@ -57,4 +57,5 @@ pub const locales = [_][:0]const u8{
     "lv",
     "vi",
     "kk",
+    "be",
 };