Translate UI and user-facing messages to Ukrainian (#5)

- All frontend pages, labels, notices and errors; html lang=uk, uk-UA money format
- Brand "Assistant System" in the top bar and page title
- Backend error details returned to the UI (auth, orders, receipts,
  cash registers, Checkbox/CRM/NP errors) and CLI output
- Tests updated for the new messages

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/core/config.py

@@ -53,7 +53,7 @@ class Settings(BaseSettings):
     # --- Первый администратор (только для команды bootstrap) ---
     first_admin_email: str = "admin@example.com"
     first_admin_password: str = ""
-    first_admin_name: str = "Администратор"
+    first_admin_name: str = "Адміністратор"
 
     # --- CRM ---
     crm_base_url: str = "https://optstore.exocrm.com/api/1.1/"