Adding a new locale

Once the Weblate translations for a single language have reached ~50% in coverage, it's time to add it to the application.

To do so:

1. In packages/commons look for i18n.ts and add a new entry to UNSORTED_LOCALES for the language.
2. In packages/commons look for dayjs.ts and add a mapping for the new language in DAYJS_LOADER. Sort the entire list.
3. In apps/client, look for collections/calendar/index.tsx and modify LOCALE_MAPPINGS to add support to the new language.
4. In apps/client, look for widgets/type_widgets/canvas/i18n.ts and modify LANGUAGE_MAPPINGS. A unit test ensures that the language is actually loadable.
5. In packages/ckeditor5, look for i18n.ts and modify LOCALE_MAPPINGS. The import validation should already check if the new value is supported by CKEditor, and there's also a test to ensure it. The test in i18n.spec.ts pins every locale by hand rather than iterating them, so add the new one there as well.
6. In apps/client, look for widgets/type_widgets/spreadsheet/locales.ts and modify UNIVER_LOCALES, either with a source (if Univer ships a bundle for the language) or with an explicit null to fall back to English. Every preset in SPREADSHEET_PRESET_PACKAGES must have a bundle for the locale, which locales.spec.ts checks.
7. Locale mappings for PDF.js might need adjustment. To do so, in packages/pdfjs-viewer/scripts/build.ts there is LOCALE_MAPPINGS. No entry is needed when the locale's electronLocale already names a directory under packages/pdfjs-viewer/viewer/locale.

Steps 1 to 6 are keyed by DISPLAYABLE_LOCALE_IDS, so pnpm typecheck reports each one still missing after step 1.