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/services/api/cbsErrors.test.ts

@@ -9,9 +9,16 @@ import {
 } from './cbsErrors';
 
 describe('error code catalogue', () => {
-  it('covers all 45 documented codes, with no duplicates', () => {
-    expect(CBS_ERROR_CODES).toHaveLength(45);
-    expect(new Set(CBS_ERROR_CODES).size).toBe(45);
+  it('covers all 46 documented codes, with no duplicates', () => {
+    expect(CBS_ERROR_CODES).toHaveLength(46);
+    expect(new Set(CBS_ERROR_CODES).size).toBe(46);
+  });
+
+  it('does not invite a retry when the bank withholds a balance', () => {
+    // The account exists and the bank simply will not price it. Repeating the
+    // call gets the same refusal, so the card must not offer a retry.
+    expect(isCbsErrorCode('ACCOUNT_BALANCE_NOT_AVAILABLE')).toBe(true);
+    expect(isRetryableError('ACCOUNT_BALANCE_NOT_AVAILABLE')).toBe(false);
   });
 
   it('knows the cheque book codes', () => {

src/services/api/cbsErrors.ts

@@ -17,6 +17,7 @@ export const CBS_ERROR_CODES = [
   'CUSTOMER_NOT_FOUND',
   'NO_ACCOUNTS_FOUND',
   'ACCOUNT_NOT_FOUND',
+  'ACCOUNT_BALANCE_NOT_AVAILABLE',
   'CHEQUE_NOT_FOUND',
   'STOP_PAYMENT_NOT_AVAILABLE',
   'INVALID_DATE_RANGE',
@@ -142,6 +143,15 @@ const ERROR_COPY: Record<AnyErrorCode, ErrorCopy> = {
     title: 'Account unavailable',
     description: 'That account is no longer part of your profile.',
   },
+  /**
+   * The account exists but the bank will not give a balance for it — a deposit
+   * asked of the wrong endpoint, or a product the connector cannot read. Not
+   * retryable: the identical call gets the identical answer.
+   */
+  ACCOUNT_BALANCE_NOT_AVAILABLE: {
+    title: 'Balance unavailable',
+    description: 'Your bank does not provide a balance for this account.',
+  },
   /**
    * Nothing on file for this account and cheque number. The bank keeps no
    * separate record of issued cheques, so this also covers the common case of

src/services/api/endpoints.ts

@@ -29,6 +29,16 @@ export const endpoints = {
      */
     list: '/accounts',
     mmid: '/accounts/mmid',
+    /**
+     * One transactable account's current balance, by `accountNumber` query
+     * parameter. Read live from the bank on every call — nothing is cached
+     * here, and nothing about a balance may be.
+     *
+     * SAVINGS and CURRENT only. A deposit's balance is a term-deposit concept
+     * and lives on `depositDetail` below, which is why the bank keeps the two
+     * apart rather than answering both from one endpoint.
+     */
+    balance: '/accounts/balance',
     /**
      * A page of ledger entries for one account. Takes the full account number
      * from `list` above, and an optional `fromDate`/`toDate` pair that the