Add a language-switch system with per-language files

Introduce a hand-rolled i18n layer across all three UIs, each reading its
text from per-language JSON files (English base + Polish + Danish). Nothing
in the converted screens hardcodes English any more.

- Web App (Vue): src/i18n/{en,pl,da}.json + index.js exposing t()/tSplit(),
  reactive to the signed-in profile locale. Every view, component, form and
  the status labels in lib/format.js go through t().
- API Server panel (Vue): src/i18n/ with its own localStorage-persisted
  language (the panel has no user profile) and a header language picker.
  Chrome, cards, login and API section titles translated; endpoint reference
  descriptions intentionally kept in English. Rebuilt embedded dist.
- Phone App (Flutter): assets/i18n/ + lib/i18n.dart loaded at startup,
  driven by AppSettings.locale. Nav, login, lock, dashboard, the full
  Settings panel (incl. language picker) and format.dart status labels
  translated; remaining detail screens fall back to English.

Language = the language half of the existing BCP-47 locale; the region half
still drives date/number/currency formatting. Missing keys fall back to
English, and plurals use Intl.PluralRules / Intl.plural so Polish gets the
correct one/few/many forms. Settings flags languages without a translation.

Tests updated to assert the localized (Polish) status wording; all pass.
See TRANSLATIONS.md for the format and how to add a language.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -37,6 +37,10 @@ export/import).
 - **Accounts & sessions** — JWT login, per-device active sessions with remote
   logout, profile + appearance preferences (theme/locale/date/font), email
   verification, and account deletion.
+- **Translated UI** — the interface reads its text from per-language files
+  (English, Polish, Danish today), driven by the language half of the user's
+  locale, with English as the fallback for any untranslated string. See
+  [TRANSLATIONS.md](TRANSLATIONS.md) for the format and how to add a language.
 - **Per-user ownership & sharing** — each car has an owner and can be shared with
   other users as read or write; the UI mirrors the server's access checks.
 - **Admin** — role-gated user management (create / role / reset password / delete).