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>

frontend/src/features/auth/LoginPage.tsx

@@ -37,7 +37,7 @@ export function LoginPage() {
     try {
       await login(email, password)
     } catch (err) {
-      const message = err instanceof ApiError ? err.message : 'Не удалось подключиться к серверу'
+      const message = err instanceof ApiError ? err.message : 'Не вдалося підключитися до сервера'
       setFormError(message)
     } finally {
       setSubmitting(false)
@@ -102,7 +102,7 @@ export function LoginPage() {
                 type="button"
                 className="login-toggle"
                 onClick={() => setShowPassword((v) => !v)}
-                aria-label={showPassword ? 'Скрыть пароль' : 'Показать пароль'}
+                aria-label={showPassword ? 'Приховати пароль' : 'Показати пароль'}
                 aria-pressed={showPassword}
               >
                 <svg viewBox="0 0 24 24" width="18" height="18" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round" aria-hidden="true">
@@ -116,7 +116,7 @@ export function LoginPage() {
 
           <button type="submit" className="login-submit" disabled={submitting}>
             {submitting && <span className="spinner" aria-hidden="true" />}
-            {submitting ? 'Входим…' : 'Войти'}
+            {submitting ? 'Входимо…' : 'Увійти'}
           </button>
         </form>
       </div>