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/schemas/receipts.py

@@ -74,7 +74,7 @@ def _masked(encrypted: str) -> str:
     try:
         return crypto.mask(crypto.decrypt(encrypted))
     except crypto.DecryptionError:
-        return "не расшифровывается — введите заново"
+        return "не розшифровується — введіть заново"
 
 
 class CashRegisterOut(BaseModel):