Add account balance enquiry via GET /accounts/balance

The CBS API now exposes a balance endpoint, which it previously did not.
Several comments in the codebase asserted that no such endpoint existed
anywhere in the API; those are corrected here rather than left to mislead.

Balance is fetched lazily, per account, from the Account Details card:

- `accounts.balance` endpoint, `CbsAccountBalance` type, and a
  `getAccountBalance` query with `keepUnusedDataFor: 0` — a balance is the
  most perishable figure in the app and must not outlive its screen.
- The call is triggered by the customer, not by render. Every call reaches
  the core banking system, and a balance should not appear over someone's
  shoulder just for opening a screen.
- `supportsBalanceEnquiry` gates the control by account type only, with no
  status check: a dormant savings account cannot be debited but still has a
  balance. Deposits are excluded because the endpoint does not serve them.
- `ACCOUNT_BALANCE_NOT_AVAILABLE` added to the error catalogue as a
  non-retryable refusal, so the card states it instead of inviting a tap
  that gets the identical answer.

Also carries in-progress work already present in the tree: customer-facing
account type labels (`accountTypeLabel`) applied across the account card,
selector, mini statement, balance enquiry and MMID screens.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/modules/services/screens/BalanceEnquiryScreen.tsx

@@ -5,6 +5,7 @@ import Ionicons from '@react-native-vector-icons/ionicons';
 
 import { Card, EmptyState, InfoBanner, Loader, ScreenShell } from '../../../components';
 import { useAccounts } from '../../home/hooks/useAccounts';
+import { accountTypeLabel } from '../../home/utils/accountRules';
 import { useTheme, fontFamily } from '../../../theme';
 import type { CbsAccount } from '../../../types/cbs';
 import type { ServicesStackParamList } from '../../../navigation/types';
@@ -38,7 +39,7 @@ export default function BalanceEnquiryScreen(_props: Props) {
         </View>
         <View style={styles.accountCopy}>
           <Text style={[styles.accountType, { color: theme.colors.primary }]}>
-            {`${account.accountType.toUpperCase()} ACCOUNT`}
+            {accountTypeLabel(account.accountType).toUpperCase()}
           </Text>
           <Text style={[styles.accountNumber, { color: theme.colors.textPrimary }]}>
             {`XXXX ${account.accountNumber.slice(-4)}`}